"I've heard the best recommendation is a tablet + USB LTE dongle, to put some space between the two processors "
This is an interesting, and tempting, direction to go in ... my understanding is that this is problematic, however.
You see, in addition to all of the (radio stuff) that the baseband processor handles, it turns out that they also handle a lot of voice quality functions, such as noise cancel, echo cancel, interference, etc. - all things that we take for granted on all phones.
I have been told that VOIP apps running on non-mobile-phone "handsets" (like you're suggesting) are somewhat difficult to use for plain old voice, because they lack all of these functions which are difficult to replicate (and are wrapped up in a lot of patents and trade secrets, etc.)
The open source PJSIP (which I do have first hand experience of) has echo cancel and some noise cancellation. It's not cutting edge but it works fine in practice. You might not get quite the quality of Skype but better than plain old landline telephone service.
2) An example of the kinds of things one has to deal with: many 8 numbers will drop calls that don't provide a supervised signal to the public switched telephone network.
Many phone apps simply neglect to implement call supervision, which causes weird failures when interfacing with some pbx systems.
A dongle has a binary blob, but it's limited to the dongle itself -- it won't most likely be able to transverse the USB pipeline and get access to system memory / processes unless there are vulnerabilities in the USB transfer itself. I can also remove the dongle from the devise and know that baseband is off -- not so if the processor is on the phone itself.
This is an interesting, and tempting, direction to go in ... my understanding is that this is problematic, however.
You see, in addition to all of the (radio stuff) that the baseband processor handles, it turns out that they also handle a lot of voice quality functions, such as noise cancel, echo cancel, interference, etc. - all things that we take for granted on all phones.
I have been told that VOIP apps running on non-mobile-phone "handsets" (like you're suggesting) are somewhat difficult to use for plain old voice, because they lack all of these functions which are difficult to replicate (and are wrapped up in a lot of patents and trade secrets, etc.)
I have no first-hand experience, however.