Hi!
I have a MP server with family and friends where we use some datapacks to enhance our experience, like your World Generation datapack, but, unfortunately when I tried to generate the world in 1.18 Pre-release 7, it didn't neither convert old chunks on y<0 nor generate new chunks to match with the old ones on top of the world.
Is it meant to be? Do you not intend to make a way for it to work on worlds with datapack?
World: https://mega.nz/file/6hpwlR5Q#c_GCg6e1cm5ThHY0XqjrGqdUbJr0D7aze0Zr6Ixmh3A
Nope.