For myself I use FastMail and there is nothing I can complain about, however I wasn't really fond of migrating towards it, since that'd require 2 separate organizations, which would pretty much beat the point (pricing would be similar, but it takes some time).
Few random thoughts about whole thing: - Fastmail is OK, there's nothing good or bad about it, however if someone is GMail user (with their fancy UI and filtering) they will be disappointed. One can add Sanebox but that's additional cost.
- I'd stay away from Protonmail for two reasons - first is they don't really have IMAP (as it's encrypted etc.), so it's either hack your own bridge or use their applications. Second reason is that I've worked with few organization which perceived protonmail-based e-mails as spam and phishing (it's super easy to create anonymous e-mail account there so...)
- iCloud+ provides custom domain support, it's kind of interesting how it works, and might save you few bucks, but the migration process is not exactly Apple-like
- Migrating away is going to take a lot of effort. When I made decision I noticed that my DNS TTL were set to 604800. I set that domain LONG time ago, so I suppose those were recommended values back then.
- Either forget about archives or be prepared for a long time to sync it. Google allows export (and that's pretty much the only way as there is 2500MB daily download quota for IMAP) but getting it uploaded is something else.
- iCloud is funny because you can't really admin it - you add and setup domain and when you share it within family it's theirs to setup, one cannot make overreaching management
For my 2 users I'm going for iCloud, cause they are already in the ecosystem quite deep, as paying customers and also it integrates nicely with other services.