One thing I've tried to find for a few years but couldn't was a version of Android that focuses on being offline as much as possible. A stable build that allows the installation of APKs and can receive calls but has no wifi and (to the greatest extent possible) is incapable of internet access. A phone with no wifi and no built in browser would probably suffice, you could then control which apps you can access by whic…
You can do it. I've built custom ROMs from the lineage code base that stripped out Bluetooth, GPS, phone, messaging, etc. It's not that hard, just time consuming.
Almost certainly beyond my skill level but outta curiosity, is disabling data (meaning all uses of the network other than fundamental things like texts and calls) something that could be done? I suspect probably not