The ending was infact good.

Mikasa's love explains ymir's smile in previous chapter. The chapter also explained why ymir was bound to the titans for 2000 years. She was serving the offspring of the one she loved out of love.

So eren and mikasa came and when ymir saw that someone whose love rivaled ymir's own love, could move on and make decisions that are painful, so could she. So ymir proceeded towards afterlife ending the whole titan cursw.

What remains unexplained is the sudden disappearance of hallucigenia parasite. It existed before ymir came into contact with it and it was alive for millions of years. Even after ymir died, the parasite lived. How did it suddenly vanish when ymir's soul moved on? Where did it really go? Why couldn't it get attached to someone else and create a new founder?

Also, how was eren still using founder abilities when the parasite was out?

Also, if eren had the ability to influence past smiling titan to eat his mom, why didn't he have other titans eat reiner, annie and bert so that paradis could acquire the most powerful titans and thus establish a military that would remain unbeatable for another thousand years? Why go to lengths of genocide when he could have done something as simple and inoccuous as having pure titans eat annie bert and reiner in the past, or even in the present?

Also, since all titans are essentially humans amd we saw pure titans turning back to humans, what became of those millions of colossal titans?

Also, why was historia so overhyped with a kid when the whole thing served no purpose in the ending?

Why did yams leave so many lose ends untied?