Another mailbox.org user here. I did the same switch as @[email protected] around the same time and I can highly recommend as well. Setting up custom domains is also not hard and well documented in their knowledge-base. I am also using it for calendars and contacts with no issues at all. A plus of their premium (3€/month) plan, apart from custom domains, is that you have access to a series of other things (appointments, videoconferencing etc.) which are a nice thing to have if you need them (as an occasional teacher in academia I enjoyed having the options, especially since I could avoid Google/Microsoft stuff).

The only annoying thing is how they handle 2fa login on their website. I rarely need to login, but when I do I always suffer.

Tuta, Proton, Murena, Nextcloud Mail, or use disposable mails like Maildrop or Altmails.

I am using startmail at the moment with a custom domainand am pleased but I do plan on migrating due to the cost for adding more mailboxes. So I am reading along here but from my research recently I personally also found Tuta attractive along with mailbox for their price and feature set. What has proton done by the way? I have never really trusted the organization but has something happened recently?

Consider one that supports MTA-STS. DANE is a plus but not widely deployed.

If EU, you have a lot of good choices. US providers are limited.