@InvictusSlayer I can't thank you enough! This was just what I needed. Thanks for the proper explaination as well, you are the best.
Now that my trees can finally grow, I'm stuck with only one issue. The leaves don't have correct colors. I made them grey like the default tree textures and was hoping I could find a few to make them colored, but I haven't found something yet. Do you happen to have an example of that as well?
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [radon.mixins.json:chunk.light.MixinLevelPropagator] from phase [DEFAULT] in config [radon.mixins.json] FAILED during APPLY
It is just mentioned in the log - check for the "Caused by" lines
https://gist.github.com/InvictusSlayer/a8acb91b89d844d95f607327896abbd6
Here's an example from my mod where I added Aspen trees. You can replace the values with the foliage, leaves, logs, trunk and any numbers you may want to change. Make sure your file is in "resources/data/modid/worldgen/configured_feature" and name it the same as your key ("sequoia.json").
The DeferredRegister and bootstrap() methods you added are redundant but other than that you're all good - the DeferredRegister is how you would store an object but that isn't necessary here since the game calls the tree instance from the data (in your case `SEQUOIA_KEY`); and bootstrap is the method that your data generator would call hence unneeded here.
Essentially Minecraft does not register features like trees (along with others) the same way it does with blocks and items.
I would highly recommend using datagen as that bootstrap method would just make the "sequoia.json" file for you. Besides that nice job and keep at it