Problem is... if you went far enough with hardware and software it would be far too threatening for the networks to allow such devices and so they would not. So then you'd also have to somehow launch and maintain an alternative global mobile wireless network.

Gotta say that I am extremely happy with a Pixel device (6a) running Graphene OS. I was expecting to have to make a lot of awkward compromises but I have not had to do that. My use-cases are probably atypical but I've been able to set up this phone to do anything and everything that I want and need to do with it and altogether it honestly works better - a lot better in every way that matters to me - than my prior Samsung-on-Android phone that it replaced.