i've been using Proton since it started. they have been very slow with the Android app development and they still do not have push notifications without GPlay. other than that, i have no complaints. recently Proton released a major update to the Android app so my first point is now moot. (PS i can't recall any time when Proton Mail has been down, for me at least)
Tutanota is another provider that i tried it before i settled on Proton. the client was very bare bones but that was a long time ago.
Mullvad might have something. i haven't looked at them in a long time.