Radio firmware doesn't actually matter much in practice because they mostly hang off a non-DMA peripheral bus, and in any case, even if that's too much for your

Radio firmware doesn't actually matter much in practice because they mostly hang off a non-DMA peripheral bus, and in any case, even if that's too much for your liking, rest assured any actual secure communication device will treat the radio as compromised by design. It comes with the territory of secure encrypted communication; the difference between "compromised radio" and "compromised router" and "malicious upstream ISP" are all effectively zero in terms of design. You use E2E, secure key storage, etc. It's all effectively the same threat model of a spy who can watch at some communication junction. Don't get me wrong, it'd be nice to have a simple modem you could put Yocto on and then attach to some other thing over a memory interface. But it's really just a dumb set of pins you read and write to; the "meat" of the device security happens outside of it, and long before the data hits it. Also I think there is some work on reverse engineering the AT commands for some PinePhone modems, which means you can install a Linux distro on the device to manage and talk to the radio. It's not the same as FDA-approved baseband firmware or anything (that wouldn't run Linux), but it's probably as close you can get for now.