I agree that MCreator should implement a better way to add custom trees, but I think it should be part of the biome editor where you could import structures as tree structures [similar to how schematics worked back when that was a thing I think] and from there have the ability to select 1 or more of those tree structures to spawn in the custom biome and additionally be able to configure the spawn weight of each tree type. Custom saplings are already simple enough, and even though checking for a 2x2 square does get a bit trickier, it's still very much possible to do without custom code.
I agree that MCreator should implement a better way to add custom trees, but I think it should be part of the biome editor where you could import structures as tree structures [similar to how schematics worked back when that was a thing I think] and from there have the ability to select 1 or more of those tree structures to spawn in the custom biome and additionally be able to configure the spawn weight of each tree type. Custom saplings are already simple enough, and even though checking for a 2x2 square does get a bit trickier, it's still very much possible to do without custom code.