I would suggest Fastmail. They are excellent in what they provide. Just email and calendar. So if you are using proton other features (pass, drive..etc) then it wouldn't be 1:1 replacment.

My concern with them is that they don't store email encrypted at rest and being an Australian company their laws allow the government to put back doors in and are not obliged to tell their clients.

The government law is out of their control but not having your emails at rest unencrypted.

What's your source on them not encrypting emails at rest? They claim they do: https://www.fastmail.com/features/security/

I am terribly sorry. I got this wrong.

It is end to end encryption they don't support as this article explains. Yes, it is two very different things however just as important.

In the past few years I've tried Proton, Mailbox.org and now I'm with Posteo.de. Unfortunately they don't support custom domains, but I use SimpleLogin to solve this problem. Overall I'm pretty happy with this setup.