I'm moving a few domains' email accounts from one supplier (Siteground, their recurring fees are huge) to another (Fastmail, or maybe Office 365, [ProtonMail only allow multi-domains on their expensive professional account]) but I'm not looking forward to moving about 20-50 accounts, per domain, from one email system to another [the domains are staying put, web hosting is being handled separately, just the email accounts are moving].

Is there an easier way? Do any providers support it?

I'm not relishing manually setting up even just a handful of domains with lists of redirects and mailboxes for all my family members and the couple of small businesses I've been managing. If I decide a month down the line it's not working, it seems a lot to go through to move all the "domains" again (i.e. setup email accounts at another provider that use my domains and have named email addresses -- eg person.1@example.com, person.one@example.com, nickname1@example.com, ... etc., and then all those for example2.com, example3.com, example4.com, each set of email addresses going to a single person).

There seems to be a lot of lock-in because of the difficulty of moving email accounts. Am I doing it wrong? Should I just relent and try and use catch-alls and filter it client side? Any advice?