I recently did the transition away from Gmail and it was a toss up between Posteo and Mailbox.org. I wanted to use my own mail clients so that eliminated Tuta, which was also more expensive. I heard that Proton email addresses aren't accepted as valid by many sites as well.

I went with Mailbox over Posteo just because I preferred the domain name and could get a decent address from it with my name. So far, it seems very solid. Although the interface of their web portal is very... German. The advanced features are frustratingly not user friendly to set up and they don't explain well what some of the settings really do but once you figured it out, it works. Since I use third party clients at least I don't have to interact with that interface often now that it is set up.