What about Mailbox.org? They have an option to automatically decrypt the mailbox (including all incoming emails and the sent folder) with your public PGP key, which essentially is the same thing thing that Protonmail does.
Protonmail requires you to use their apps on mobile, and their "bridge" on desktop to use it with a normal email client. And their web client uses Javascript to decrypt the emails in the browser.
Mailbox.org on the other hand allows you to use any email client that supports PGP (e.g. K-9 Mail + OpenKeychain on Android, Thunderbird or KMail on Desktop) to download the encrypted emails and locally decrypt them on your device, and for the web client they are compatible with the Mailvelope extension to decrypt your emails in the browser.
I kind of prefer Mailbox.org's approach as it is more "open" with no requirement to use their app. Protonmail on the other hand is all set up out of the box with no thinking and tinkering needed. I think if Proton would offer their bridge for Android, I would prefer them but until then I'm leaning towards Mailbox.org because of their openness.