Assuming empathy is a worldly thing (which i believe it is), why does/would a buddha try to help others find the way?

Perhaps you're assuming a syllogism like,

1. Empathy (and cooperation) is a biological feature of social animals which (via evolution of the species) benefits the self (or benefits the species, or the world) by benefiting one's allies
2. The Buddha is independent and requires no allies
3. Therefore the Buddha need not experience empathy, nor practice it

I guess that's a selfish and self-serving view: which you should hesitate to ascribe to the Buddha as a motive.

Another motive might be conceit (e.g. "I'm more enlightened than you") but we're told that the Buddha is beyond that too.

I assume instead that it may be motivated some kind of kindness or generosity. Or it may be some kind of virtue -- virtuous behaviour.

I think there's a sutta where the Buddha says, something like (I don't remember it well), that a person should venerate their teacher ... that being somehow subservient or obedient (to the teacher) is essential ... and that the Buddha himself, with no teacher of his own, said that he would obey the Dhamma as his own teacher.

I think that even the precepts, i.e. "don't kill" and so on, are based on empathy/morality/virtue.

Also though "unselfish empathy" may be a heavenly attitude, it's only one of them -- there's also "taking pleasure in another's virtue" (which might be another reason perhaps for the buddha's encouraging others' enlightenment), as well as equanimity (see "brahmaviharas" for more detail).

Another answer is that there are people who do as you suggest, i.e. attain enlightenment but without being willing or able to teach; these people might be known as "arhats" or "private Buddhas" -- so (trivially) the Buddha tries to help by definition, i.e. if he didn't try to help he wouldn't be a Buddha, it's his trying to help that makes him a Buddha.

I believe the answer to your question is given in "The Brahma's Request" quoted below.

As it reads, the Buddha taught out of compassion / mercy to beings almost free from the dust of worldlings.

The Buddha refrain from passing into the final Nirvana after a request by Brahma Sahampati.

THE BRAHMA'S REQUEST

The Blessed One having attained Buddhahood while resting under the shepherd's Nigrodha tree on the banks of the river Neranjara, pronounced this solemn utterance:

"How sure his pathway in this wood, Who follows truth's unchanging call! How blessed, to be kind and good, And practice self-restraint in all! How light, from passion to be free, And sensual joys to let go by! And yet his greatest bliss will be When he has quelled the pride of 'I'.

"I have recognized the deepest truth, which is sublime and peace-giving' but difficult to understand; for most men move in a sphere of worldly interests and find their delight in worldly desires. The worldling will not understand the doctrine, for to him there is happiness in selfhood only, and the bliss that lies in a complete surrender to truth is unintelligible to him. He will call resignation what to the enlightened mind is the purest joy. He will see annihilation where the perfected one finds immortality. He will regard as death what the conqueror of self knows to be life everlasting. The truth remains hidden from him who is in the bondage of hate and desire. Nirvana remains incomprehensible and mysterious to the vulgar whose minds are beclouded with worldly interests. Should I preach the doctrine and mankind not comprehend it, it would bring me only fatigue and trouble."

Mara, the Evil One, on hearing the words of the Blessed Buddha, approached and said: "Be greeted, thou Holy One. Thou hast attained the highest bliss and it is time for thee to enter into the final Nirvana."

Then Brahma Sahampati descended from the heavens and, having worshiped the Blessed One, said: "Alas! the world must perish, should the Holy One, the Tathagata, decide not to teach the Dharma. Be merciful to those that struggle; have compassion upon the sufferers; pity the creatures who are hopelessly entangled in the snares of sorrow. There are some beings that are almost free from the dust of worldliness. If they hear not the doctrine preached, they will be lost. But if they hear it, they will believe and be saved."

The Blessed One, full of compassion, looked with the eye of a Buddha upon all sentient creatures, and he saw among them beings whose minds were but scarcely covered by the dust of worldliness, who were of good disposition and easy to instruct. He saw some who were conscious of the dangers of lust and wrong doing. And the Blessed One said to Brahma Sahampati: "Wide open be the door of immortality to all who have ears to hear. May they receive the Dharma with faith."

Then the Blessed One turned to Mara, saying: "I shall not pass into the final Nirvana, O Evil One, until there be not only brethren and sisters of an Order, but also lay disciples of both sexes, who shall have become true hearers, wise, well trained, ready and learned, versed in the scriptures, fulfilling all the greater and lesser duties, correct in life, walking according to the precepts-until they, having thus themselves learned the doctrine, shall be able to give information to others concerning it, preach it, make it known, establish it, open it, minutely explain it, and make it clear-until they, when others start vain doctrines, shall be able to vanquish and refute them, and so to spread the wonderworking truth abroad. I shall not die until the pure religion of truth shall have become successful, prosperous, widespread, and popular in all its full extent-until, in a word, it shall have been well proclaimed among men!"

Then Brahma Sahampati understood that the Blessed One had granted his request and would preach the doctrine.

Emphasis added

I think this could be understood as the lack of ill will - so its more what a buddha wouldnt have

The opposite of many bad (unwholesome-akusala) things is good (wholesome=kusala)

so the opposite of ill will i believe be empathy - the 5 hindrances which arhants (and the buddha was an arahant) dont have :

Sensory desire (kāmacchanda): the particular type of wanting that seeks for happiness through the five senses of sight, sound, smell, taste and physical feeling. .... opposite will be to be content with what is

Ill-will (vyāpāda; also spelled byāpāda): all kinds of thought related to wanting to reject; feelings of hostility, resentment, hatred and bitterness.

.... opposite will be metta to all

Sloth-and-torpor (thīna-middha): heaviness of body and dullness of mind which drag one down into disabling inertia and thick depression. ... opposite will be enregetic

Restlessness-and-worry (uddhacca-kukkucca): the inability to calm the mind. .... opposite will be calm

Doubt ... opposite will be to have faith in the dhamma

They say Buddha's mind is incomprehensible. It is impossible to understand Buddha's perspective or motives, because it's beyond any fixed position.

Chogyam Trungpa said, enlightened mind sees everything from all possible angles, "like a lawyer". I guess you could call that quantum vision or multidimensional mind.

Because of this, it's really impossible to give one answer to "why does a buddha try to help others?". There are many reasons, but at the end of the day the answer is: "because Buddha is Buddha and this is what Buddha does, by definition"

Enlightened beings help others due to compassion. Compassion(Karuna) is a virtuous mind state. When someone becomes enlightened, he only cuts off unwholesome mind states. Virtuous mind states continue to arise.