After a lot of trial and error I finally managed to create a datapack with a custom dimension with 2 custom biomes, a custom structure and a custom tree. Placement of stuff is still a bit wonky and I haven't figured out yet how to make the structures rarer.

You can download the datapack here if you want. Use the command /execute in custom:custom_biomes run tp @s 5 63 5 to get to the custom dimension.

Note that you need to use snapshot 20w28a for this to work.

Ahoj

I notice your trees are all equidistant from eachother. I have also been having this problem. Do you know how to fix it?