https://www.reddit.com/r/feedthebeast/comments/14tjgqe/biometweaker_help/
^^^ everything is in this reddit post by me ^^^

Create output files (not in a superflat world), copy the dimension file into a datapack, delete the first line

"registry_name": "minecraft:overworld",
"dimension": {

including the closing } and delete the corresponding entries of the biomes.