Things I found out so far: you can pretty much ignore the error you get in the log, they dont help. I found it helpfull to add a breakpoint to Line 198 in WorldSettingImport (net.minecraft.util.registry) there you see the actuall exceptions, like it did not find a certain feature
You can not use vanilla jsons, only the jsons inside your datapack so you need to copy the vanilla stuff you want/need.