I would love to have this. I could make a bunch of 'small-ish' islands that contain all biomes, for each player, to see what they make of their own little worlds.

Alas, none exist that I know of.

Yea I wanted to curate a whole map with selected biomes in selected positions so you have to actually explore and expand, right now you can do most of the game in the surrounding area

Better Continents on nexusmod.com is the closest I've found so far.

I just wish they'd add features to the seed creation, I'd like to see for example one big land mass and see all the mixed areas from Biome to biome

There is a discord and an github you can goto as well :P