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

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.