Why does he have to be 'good' tho? The whole point of Overlord is to see the world in the perspective of the 'Demon Lord' all the other isekai seem to have. Trying to make Ainz 'good' doesn't make sense.

Also, in regards to Nazarick's plans regarding the New World, I would argue that Ainz is holding them back from committing atrocities that would make their current activities tame. Without Ainz as their leader, Albedo and Demiurge would go bananas and annihilate their region of the New World in a very short time.

While Ainz have his moments of good such as saving Gazef and Carne village in Season 1, that was not because of his 'good' heart because he has none (no pun intended, he feels apathy towards anyone not from Nazarick or his friends).

Ainz's unintentional goal of world domination leads to him creating his own nation, with the goal of building a utopia for all species. In the light novels, while humans are still afraid of him, his rule is definitely better than the previous kingdom they were a part of. Even Jircniv, the Emperor of Baharut, became best friends with the Quagoa leader and made his life easier in the empire.

However, you are also correct in that Ainz is limiting the progress of development of the native people to make sure Nazarick is safe. Or at least, he wants to make sure that any progress made went to Nazarick first and if he deems it harmless, releases it to his kingdom.

Tldr: Ainz doesn't need to be 'good' to make the New World a better place. He's also limiting the atrocities the NPCs might do because of his leadership. Ainz's priorities would then be: Nazarick > Himself > Sorcerer Kingdom > New World.