Get a device with a separate baseband processor which connects to the main SoC through a serial connection. Both the PinePhone [1] and the Librem 5 [2] come so equipped, both also feature hardware kill switches with which the baseband can be disabled.
Both devices are aimed at the hacker and enthusiast markets but once you're at the level of 'paranoia' which makes you suspect the radio baseband processor is being (ab)used in nefarious ways to track you this should not dissuade you from taking the leap. The more esoteric the hard- and software, the less chance there is for some cookie-cutter exploit to catch you.
If I was paranoid, I'd say those devices are developed just to get the most paranoid to use them. :P
But in truth, I'm not paranoid.. merely a realist. Regardless, I was thinking of buying a Librem, but can you even get one yet? It always says "6 months to delivery".
The design seems to work in favour of the paranoid, it takes an extra-double-ultra-plus paranoid who takes the board through an X-ray machine to find those hidden traces or sandwiched secret baseband processors to uncover this nefarious plot.
Alternatively these devices deliver on this front, i.e. the user gets to control whether the baseband is active or not. I'm not paranoid enough for the former scenario so I'll keep it at the latter and am considering to get a Pinephone for fun and, well, fun I guess - I already have plenty of devices running different iterations of AOSP, some of them running headless Linux without zygote (i.e. without the Android user space). A truly modular multi-vendor architecture with pluggable SoC, baseband, storage, audio, camera, sensor package, power supply and display would be the preferred solution - something like the original PC clone market with vendors competing on price and features - but I'm not going to hold my breath waiting for it. Not that long, anyway.
Both devices are aimed at the hacker and enthusiast markets but once you're at the level of 'paranoia' which makes you suspect the radio baseband processor is being (ab)used in nefarious ways to track you this should not dissuade you from taking the leap. The more esoteric the hard- and software, the less chance there is for some cookie-cutter exploit to catch you.
[1] https://en.wikipedia.org/wiki/PinePhone
[2] https://en.wikipedia.org/wiki/Librem_5