I've also been using ProtonMail since the beginning of last year and neither a heavy mail user, but need it still. I'm paying for it and using my own domain. Some things to consider:

1) The web UI is probably not very nice and snappy if you use email a lot.

2) No IMAP for Linux (still).

3) If you happen to be like me (a bit of an idiot sometimes) and lock your phone so you don't get access to your authenticator AND you forgot the security keys, all your old emails will be PGP garbage from now on.

4) The Android app is a bit so-so... Bugs happen.

The good parts are it works, doesn't spy on you, encrypts everything client-side and is based in Switzerland outside of US and EU jurisdiction.