I want to have villagers to sometimes sit in my world.

Maybe when they stay still they will sit as well.

This requires villagers to sit on specific blocks in order to make sense (rather than sitting on the place they currently stand). Do you think it's sufficient if they sit where the player stands?

Ideally would be actual chair for both players and villagers to sit, but still, maybe too much.

I'll see how furniture mods implement sitting, maybe I can integrate that properly.

The Minecolonies mod generates a "sitting entity", inside of block that is tagged with the appropriate sitting tag, the entity is invisible, invulnerable, indestructible and is recognized by citizens, the citizens, then come, mount the entity, which make it looks like the npc is actually sitting on the block, that js one way to handle a sitting mechanic.