In reply to Steve Grant.

I'm right there with you. That's one of the reasons I think it'd be nice to have a method for doing this via PHP, which would allow theme devs to organize and manage this in a way that makes sense to them.

Currently, I am splitting my theme.json into multiple JSON files and merging them with this webpack script, but that method makes the barrier to entry for new devs much higher. At least it keeps me sane for the moment though.