Ive been on and off with Protonmail since they started, protons should have bought a lifetime subscription when I had the chance, and have been testing out the desktop app. I feel like that’s been a game changer. Along with this and the month view in the iOS calendar app I feel like the feature set is where I want it and have gone all in with all the Proton services.
No; I have a free Proton account, but even their Proton Unlimited plan (which is more expensive than Fastmail) allows only 3 custom email domains. I currently use more (should maybe reconsider that?), so I’m not even considering to migrate away from Fastmail.
However, if Fastmail would ever screw up big time (happy customer for more than 10 years so far) Proton is first on my list.