Both Fastmail and ProtonMail are quite expensive if you need more than one mailbox (user account). ProtonMail offers IMAP only through its own bridge application, and that’s only for paid accounts. Fastmail offers IMAP without any additional software requirements.
I would suggest looking at options like Mailbox.org, Runbox, Mailfence, and a few others that I don’t recall right now.
For a pricing scheme that’s not based on number of mailboxes but on other parameters like storage and mails sent, there are options like mxroute and Migadu. I don’t prefer either of these for different reasons (the former is in the U.S. and the latter left me with a bad taste when it moved from a free plan to paid).
