I’m in the same boat as you and looking to migrate to Fastmail. I thought about Tuta but, like Proton, they don’t support migrating *away* from them via IMAP and now that I’m dealing with it, it feels a little too vendor-lock-in-y. At this point, for me anyway, standard IMAP with no hoops is table stakes.
(There’s an IMAP bridge for Proton but to use it to migrate providers is convoluted to say the least.)