(I’ll answer about my thoughts and opinions are instead of what I am actually using.) I would not be adverse to using a “feature phone” – i.e. a non-updateable non-smartphone. It’s hard coded; no one can affect the software in it, so it would be OK if I can’t either. Of course a free (RYF-certified) phone would have much better features (including inspectable privacy guarantees), which is why I’m considering such a thing.

How do you know the software can't be affected remotely, considering the baseband processor?

I can’t know that, which is why I wrote “Of course a free (RYF-certified) phone would have much better features (including inspectable privacy guarantees), which is why I’m considering such a thing.”