I was using two other systems in the past before giving up and using Android. The first was Meego by Nokia and Intel, on the Nokia N9. This is not an option anymore since Nokia switched to Windows Phone and then stopped altogether (current Nokia brand not being the same company). The second was Sailfish OS by Jolla. It can still be installed on some older Xperia phones and is a viable option. Though the development could not keep up with iOS / Android. There is some Android app compatibility. For a while I thought KaiOS might become an option, if scaled up a bit, becoming more feature rich. But I think cheap Android phones with Android One have taken that niche now.

Why not use good old signatures for that?