It occurs to me that I almost maybe have a slight contribution to the discussion. Not a phone, but this guy builds security-hardened Linux laptops from the BIOS level up. Don't trust me or him, of course, but you can easily investigate him. He has an educational charity.
https://www.facebook.com/lysander.spooner.7/posts/1366736536769431
Beats the hell out of some stock windows boxen for crypto.