You can use it with a client via POP/IMAP as per here. Seems to even come with a free app that allows end to end encryption. Doesn't look as user friendly as setting up Gmail on a client though. Also, not sure if you can load non-Proton accounts into the webmail UI like you can with Gmail.
Do you still use Thunderbird?
No, not anymore, although I hear there is a new Thunderbird UI out. Might try it just for fun. ::edit:: Downloaded the new Thunderbird and it was a easy setup with Gmail. So far liking the dark theme but still getting used to the new UI.