thecrankosaurus having used Openkeychain with what was K9 mail and now Thunderbird for Android, I found it easy to use and efficient. I have since changed to Tuta mail so openkeychain has been removed. For me PGP is far too complicated for people to bother with, even people used to PGP can at times find themselves in error. Many email suppliers implement PGP themselves, mailfence and mailbox come to mind, I would guess most users dont even bother/think about PGP. Protonmail with its app has PGP wrapped up inside it and my favourite Tuta does its own thing without PGP, even Thunderbird has can use PGP without add ons etc, so I think the userbase for Openkeychain gets smaller every day.
i still think PGP is a viable security asset, it has to be used correctly, just like GrapheneOS does..