[throwaway81523]

I have sometimes thought of ditching my phone plan and getting a wifi hotspot, just to stop the phone carrier from messing with the software in my phone through OTA updates and whatnot. All communication would be through TCP and that would stop the phone carrier from talking to the mobile baseband processor, which could be completely disabled or removed. It would even allow ditching the whole phone and using a wifi-only tablet instead. Both sides (the wifi box and the phone) are hard to make entirely free, but by isolating them from each other, some higher control can be achieved.

[aesh2Xa1]

Another HN user, tptacek, has made comments going back years now that point out how modern Android (at least Pixels) and iPhones all isolate the baseband behind a serial/USB peripheral interface. I'm not sure you would gain anything at all by going with your surmised setup above.

[tata71]

Many carriers do OTA config stuff when you insert their SIM,

cooperated with by your carrier.