Stages of equanimity and the end

Thanissaro Bhikkhu states that the Buddha said Nibbana is the highest happiness.

From the suttas, we have:

SN46.54:15.10: The apex of the heart’s release by equanimity is the dimension of nothingness, I say, for a mendicant who has not penetrated to a higher freedom.”

Equanimity corresponds with the sixth liberation. There are two more:

DN34:2.1.202: Going totally beyond the dimension of infinite consciousness, aware that ‘there is nothing at all’, they enter and remain in the dimension of nothingness.
DN34:2.1.203: This is the sixth liberation
DN34:2.1.204: Going totally beyond the dimension of nothingness, they enter and remain in the dimension of neither perception nor non-perception.
DN34:2.1.205: This is the seventh liberation.
DN34:2.1.206: Going totally beyond the dimension of neither perception nor non-perception, they enter and remain in the cessation of perception and feeling.
DN34:2.1.207: This is the eighth liberation.

It may also be interesting to read MN43:

MN43:29.5: “The limitless release of the heart, and the release of the heart through nothingness, and the release of the heart through emptiness, and the signless release of the heart: do these things differ in both meaning and phrasing? Or do they mean the same thing, and differ only in the phrasing?”

As i understand, "The dimension of nothingness" is an apex of equamity in as far as perception attainments go and "The dimension of neither perception nor non-perception" is an equanimity highest in as far as sustenance goes.

"There is the case, Ananda, where a monk, having practiced in this way — (thinking) 'It should not be, it should not occur to me; it will not be, it will not occur to me. What is, what has come to be, that I abandon' — obtains equanimity. He relishes that equanimity, welcomes it, remains fastened to it. As he relishes that equanimity, welcomes it, remains fastened to it, his consciousness is dependent on it, is sustained by it (clings to it). With clinging/sustenance, Ananda, a monk is not totally unbound."

"Being sustained, where is that monk sustained?"

"The dimension of neither perception nor non-perception."

"Then, indeed, being sustained, he is sustained by the supreme sustenance." Mn106

Yet - equanimity doing the highest factor of the factors for awakening - does this factor then results in a further kind of equanimity or does equanimity fade into something else, once highest, aim, is reached?

Per MN 54, there're different grades of equanimity depending on where one is on the Path:

“So too, householder, a noble disciple considers thus: ‘Sensual pleasures have been compared to a skeleton by the Blessed One; they provide much suffering and much despair, while the danger in them is great.’ Having seen this thus as it actually is with proper wisdom, he avoids the equanimity that is diversified, based on diversity, and develops the equanimity that is unified, based on unity, where clinging to the material things of the world utterly ceases without remainder. ~~ MN 54 ~~

Does equanimity evolve into something else? What's higher than equanimity?

Questions of King Milinda say:

As the rafters of a house connect up to the ridge-pole, and the ridge-pole is the highest point of the roof, so do good qualities lead up to concentration.

What is meant here by concentration? Certainly not staring-at-the-candle-until-eyes-pop-out kind. Instead, it is a kind of mindfulness or collectedness maintained throughout all activities. This type of collectedness enables continuous suchness by preventing any and all mental disconnect between "what is" and "what is on one's mind". In other words it's a kind of active dynamic concentration that continuously maintains the mind in sync with things-as-they-really-are.

If you ever meet an enlightened person, you'll see how they move with a kind of royal grace and precision. It's like they are always comfortable, never losing their center. This royal grace and precision are outer expressions of the right concentration.