Is nirvana just a checkpoint in an eternal journey?

Nirvana, according to Buddha, is the ultimate treasure for Human and any kind of lives, surpassing all other forms of satisfaction. To illustrate this, consider the peak satisfaction during sexual pleasure akin to the mesocarp of a coconut, while Nirvana's contentment is akin to the innermost endocarp and coconut water. However, there exists no comparison or illustration that can truly encapsulate the profound contentment Nirvana bestows. Buddhism teaches that Nirvana can be attained by transcending ego, hatred, desire, and bonds, and comprehending the universe's truth. After Nirvana, there is no rebirth – no body, no soul – akin to a lamp's flame extinguishing, its destination unknown. Achieving Nirvana is breaking the cycle.

Given the Four Noble Truths, I think that the suttas explains nirvana as permanent or perpetual non-arising of suffering, following a complete uprooting of desire and craving (including any craving for further material or immaterial existence), including of their causes (e.g. asavas meaning "outflows" and "intoxication").

Metaphysically (I'm pretty sure it's right to say that) it is not "temporary" at all -- because it's not created, so it cannot be uncreated (see this answer for a partial explanation or small example).

One bit of Mahayana (Tibetan) doctrine on that kind of subject that I read is quoted in this answer -- which I think says that there is a "not the final nirvana" which is manifested by the Buddha as a "resting place" and that after a while "Arhats" are awoken by the Buddha and "practice the bodhisattva’s path for many limitless kalpas and eventually achieve enlightenment."

I'm not saying that Mahayana contradicts the Theravada, perhaps it sounds like they do.

I think both schools have a doctrine of gradual enlightenment -- Theravada's is outlined here for example. I'm not sure it makes sense to say that even a first stage is temporary, it's described using words like "complete".

What lies beyond our universe? Are there other universes? Not like that of a multiverse but other universes that evolved just as ours evolved. What if these universes evolve at different rates and are in different stages of their evolution. Will we ever know them, how they operate, the beings in them? And if we travel beyond our universe, will our body (the atoms, sub-particles and whatever that makes up matter) and mind be freed from the bondage imposed by our universe?

Once, a highly accomplished seer named Rohitassa attempted to travel to the end of the cosmos (or universe) to find out if birth and death ends once beyond. He failed and was reborn as a deva. After he related his story, the Buddha declared:

But at the same time, I tell you that there is no making an end of suffering & stress without reaching the end of the cosmos.

So maybe there are even more amazing things waiting to be discovered after nirvana (or parinirvana). But this is only possible if we are willing to free ourselves from our existing cage. I suspect our consciousness in its current form is just unable to fathom or traverse the sheer immensity of what lies beyond. Perhaps, this is why a new consciousness is needed; one that does not land anywhere or increase (and decrease) as alluded in Atthi Raga Sutta.

In any case, who says this existing cage has only amazing things to offer? For one, there’s illness. When we are sick, amazing places like hills, mountains and what not are just not that enjoyable. It is not the places, people and things that brings us joy, it is what is inside us. Besides, after immeasurable rebirths, have we not tasted enough of this universe? With Metta.

You sound like someone from the Deva realm :) I guess you've had many good experiences on this earth, but you also have to remember that there is a lot of suffering in samsara.

Devas in heavenly realms are living in bliss, and therefore they have less motivation to seek the Dhamma, since they barely experience any suffering. But once their good karma runs out, they will be reborn somewhere else, and it could be in one of the hell realms as well. In the 31 planes of existence (https://i.redd.it/ruc3w8ej7x381.jpg), you're only safe from the hell realms once you get reborn in Suddhavasa realms.

A large number of Devas attained enlightenment during the Buddha's time after listening to his sermons. So, I think that for you, you're having a wonderful life because of your good karma. But don't forget that that too is impermanent, so don't ever shy away from doing 'good merits' so that you will get an opportunity to attain Nibbana (since by doing good merits, you have more of a chance to be reborn in a realm where you are exposed to the Dhamma).

You can be mindful about the fact that there's an vast amount of suffering in the world, and use that knowledge to motivate you to pursue the Dhamma so that you can work towards liberating yourself from any possible karma that may arise to cause you any suffering.

The Buddha said to practice putting an end to the cycle of birth and death without any delays - as if your head was on fire (https://suttacentral.net/sn4.9/en/bodhi?reference=none&highlight=false).

I cant find this reference, and I'm saying this story from memory. Once a monk had attained either sotapanna, sakadagami or anagami, and then he thought "I'm almost there now, I might just relax for sometime before attaining enlightenment". And in the time that it takes to bend your arm, the Buddha was there. The Buddha said, if there was even a little bit of faeces on your clothes, would you not find it disgusting? So this is like that too. You need to practice and attain enlightenment right now, so that you are fully released from suffering.

Edit: Thought I'd add - I remember as a young kid when my mum was talking about Nirvana, and how it's the end of the cycle of birth and death, I asked her why you wouldn't want to be born again. My mum said because to keep being born and dying is suffering. I didn't understand it at the time since I was having so much fun being a kid that I wanted to keep being born to enjoy it. I feel the complete opposite way now though at 30 years old and can't wait to not be born again!

I am expecting more from the universe. I just can't believe nirvana is the end of it all.

And that, sir, is your problem. You’re expecting, looking, seeking. That’s going to drive you batty after awhile. The ability to put that kind of searching aside is what pushes us in the direction of liberation. So as long as you have the idea that something, somewhere out there is worth seeking, you are walking away from nirvana.

And not for nothing, but don’t you think you’d have to have experienced enlightenment to make that assessment? Just sayin’.

Case 29 of the Blue Rock Collection
The Kalpa Ending Fire

A monk asked Dasui, "When the kalpa fire flares up and the great cosmos is destroyed, I wonder, will "it" perish, or will it not perish?"
Dasui said, "It will perish.
The monk said, "Then it will be gone with the other?"
Dasui said, "It will be gone with the other."

What is it? Why are you chasing it?

Ven. Yamaka expressed a similar type of view i.e. speculating whether the Buddha exists or doesn't exist or both exists and doesn't exist, or neither exists nor doesn't exist.

The sutta below clearly explains how it should be, according to the teachings found in the Early Buddhist Texts (the Theravada suttas and Mahayana agamas).

The question is not whether the journey continues or not. In reality, there was never a journey or a being or a person in the first place that's concrete and standalone.

All that was, is suffering that came to an end after Nirvana.

Then, in the evening, the Venerable Sāriputta emerged from seclusion. He approached the Venerable Yamaka and exchanged greetings with him, after which he sat down to one side and said to him: “Is it true, friend Yamaka, that such a pernicious view as this has arisen in you: ‘As I understand the Dhamma taught by the Blessed One, a bhikkhu whose taints are destroyed is annihilated and perishes with the breakup of the body and does not exist after death’?”

“Exactly so, friend.”

“What do you think, friend Yamaka, is form permanent or impermanent?”—“Impermanent, friend.”…—“Therefore … Seeing thus … He understands: ‘… there is no more for this state of being.’

“What do you think, friend Yamaka, do you regard form as the Tathagata?”—“No, friend.”—“Do you regard feeling … perception … volitional formations … consciousness as the Tathagata?”—“No, friend.”

“What do you think, friend Yamaka, do you regard the Tathagata as in form?”—“No, friend.”—“Do you regard the Tathagata as apart from form?”—“No, friend.”—“Do you regard the Tathagata as in feeling? As apart from feeling? As in perception? As apart from perception? As in volitional formations? As apart from volitional formations? As in consciousness? As apart from consciousness?”—“No, friend.”

“What do you think, friend Yamaka, do you regard form, feeling, perception, volitional formations, and consciousness taken together as the Tathagata?”—“No, friend.”

“What do you think, friend Yamaka, do you regard the Tathagata as one who is without form, without feeling, without perception, without volitional formations, without consciousness?”—“No, friend.”

“But, friend, when the Tathagata is not apprehended by you as real and actual here in this very life, is it fitting for you to declare: ‘As I understand the Dhamma taught by the Blessed One, a bhikkhu whose taints are destroyed is annihilated and perishes with the breakup of the body and does not exist after death’?”

“Formerly, friend Sāriputta, when I was ignorant, I did hold that pernicious view, but now that I have heard this Dhamma teaching of the Venerable Sāriputta I have abandoned that pernicious view and have made the breakthrough to the Dhamma.”

“If, friend Yamaka, they were to ask you: ‘Friend Yamaka, when a bhikkhu is an arahant, one whose taints are destroyed, what happens to him with the breakup of the body, after death?’—being asked thus, what would you answer?”

“If they were to ask me this, friend, I would answer thus: ‘Friends, form is impermanent; what is impermanent is suffering; what is suffering has ceased and passed away. Feeling … Perception … Volitional formations … Consciousness is impermanent; what is impermanent is suffering; what is suffering has ceased and passed away.’ Being asked thus, friend, I would answer in such a way.”
SN 22.85

Nirvana is not the end, although from the perspective of a mind bound inside the illusion of time and space, it will appear that an end is arriving. As such, the mind will ask yearning-based questions, even at the mere thought of nirvana.

My understanding of Buddhism, particularly from the Theravada tradition, is that the entire human consciousness is seen to be an illusion. This is generally framed using the 5 aggregates, from which an experiencer feels they are having a true and real experience situated inside the body. That experiencer feels like they are looking out through the eyes into a real world, but even the concept of an outside world is an internal conception.

One later sees that the aggregates are not-self. This is sometimes referred to as being in the witness phase. It is a type of effortless mindfulness. Later though, something quite astonishing happens, whereby the aggregates themselves completely vanish, and my goodness, do they vanish! This is the meaning of nirvana - to be completely blown out!

There is no centre to experience, and there never was. Therefore, the ending is of this very illusion, but since it is an illusion, thereby not being real, nothing was ever realized. In the Mahayana tradition, this was emphatically delivered in the Diamond sutra:

Subhuti again asked, “Blessed lord, when you attained complete Enlightenment, did you feel in your mind that nothing had been acquired?”

The Buddha replied:

“That is it exactly, Subhuti. When I attained total Enlightenment, I did not feel, as the mind feels, any arbitrary conception of spiritual truth, not even the slightest. Even the words ‘total Enlightenment’ are merely words, they are used merely as a figure of speech.”

So these words aren't true - they are only meant as figures of speech. What remains can be so breathtaking, it stuns the mind into a deadening silence. The world becomes a mashable kaleidoscope of colours, shapes, and noises, and the body becomes the environment. One keeps going, in a sense, but you don't know what is moving you any more. Perhaps the words unconditional love fits best; one is moved by a love for everything.