> SMS/SIP features of the phone were actually on separate hardware... basically a phone that only uses SDR for connectivity?

Not exactly what you’re after, but the Librem 5 (crowd-funded, at the head-scratching stage) aims to put the baseband processor in an optional M2 slot with killable power. On a device that runs Debian. From following progress on that project I’ve got the impression that everything to do with chips for mobile connectivity are so patent/IP-law encumbered that there’s little hope for Free Software replacements.

https://puri.sm/shop/librem-5/

However, hacking on that handset is the best chance I’m aware of for you getting the device you want (perhaps with something else in the M2 slot).