Stop switching to every new “cool” and “encrypted” service every time the previous one disappointed you a little bit. This is not helpful neither for your time, nor for privacy community overall. We should lean on most developed and best platforms we have right now (Proton, Signal etc.) and get everyone to switch. Unless we know we have market dominance, dispersing our efforts is futile.

And yep, answering your question, Tuta Mail is fine

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Graphene OS - best privacy and security, only for Google Pixel phones

Calyx OS - less secure but supports cheaper phones (Motorola) and eco-friendly phones (Fairphone)

Honorable mentions are LineageOS, /e/ OS, Replicant and iode OS, however their security is much worse than of Calyx/Graphene (no verified boot), so use at your own risk

Yes, they exist, but there is no reason to use them other than tinkering around. Also they have much worse security than Android or iOS. So if you need something private and open source there are a plenty of degoogled Android ROMs.