Now that I've changed my email provider to one that isn't proton, and set up end-to-end encryption in #Thunderbird I honestly would highly recommend Thunderbird. It's the first time I've used it in many years. The UI looks modern, and it makes it super obvious if you're actually getting E2EE. It also automates key sharing and key lookup.
This is all built-in btw. No extension needed, it just does OpenPGP as standard.
Proton mail never actually did this in a way I was satisfied with. Now I actually know if the email I'm sending is going to be encrypted before I send it.
I might start adding a little message to the end of any unencrypted mail I send explaining that I was unable to find a suitable encryption key, see if I can subtly encourage people to start setting up E2EE. 
Oh btw Thunderbird works with Gmail and so you can stop Google reading your emails by setting this up. (People and websites will continue to send you unencrypted messages though, those ones they could still read)
@lexbailey I switched from Proton to Migadu which lets me have a catch all email and I can send emails easily as any address on my domain. It's my first time using Thunderbird too and it's really nice.