Oh my approach here was SO WRONG!
Somehow I’ve gotten along just fine for so long without knowing about acf_add_local_field_group()! Instead of juggling json files and synchronization, all I had to do was rewrite this to build the field groups from code in the first place. Once that function clicked, this all fell into place.
https://www.advancedcustomfields.com/resources/register-fields-via-php/
