Paid email hosting for domains, but not Fastmail

I've been with Fastmail for 6 or so years. I am ready to move. Who should I consider? I'd prefer to not use Google or Microsoft, though I realise this might not be practical. I would prefer not to self-host, I just want email to work.

It appears Fastmail consider Mail.app on Catalina insecure, and it requires its own 16-character alphanumeric password to access my email via IMAP. I set a password, and tried it in Mail.app…and it didn't work. It did work in Thunderbird. I tried their pre-made .mobileconfig file to set up the account via System Preferences…and that didn't work in Mail.app.

I raised a support ticket asking for some guidance on what to do next – explicitly mentioning that the app password did work in Thunderbird – but was told my password was wrong and I have to reset it…and the ticket was closed. I've spent most of a day trying various things and I'm done. My email has been just fine for 6 years, it started getting a bit wobbly about a week or so ago, and I've lost all patience trying to resolve this. So, I'm looking for another provider.

I am looking at Protonmail, since I have no requirement for any of the Google or Microsoft productivity suites…anyone else sensible I could look into? I don't need turbo levels of privacy or suchlike, just a setup that works (that I don't have to self host, since I do a lot of that and self-hosted email is a nut I haven't cracked yet).

Thank you.