I've thinking about ditching my Protonmail email address with Posteo since I need IMAP support (because I hate using the Protonmail app) and honestly paying for Proton Mail Plus didn't worth the money because they are pushing me to buy the Protonmail Unlimited but I don't need all those capabilities nor would I use them and the best alternative I've found is Posteo and honestly I like it, I even got CalDAV support which I didn't have in Protonmail, but personally I don't know anybody who use Posteo, so I post this here.
Worked like a charm for me for the last few years—worth the €1/mo. I understand their stance on it, but I still kinda wish they allowed custom domains. Custom domains are very useful for being able to migrate later if you change your mind. There are other good options in the space but the importantest steps are leaving Microsoft/Google/AOL/Yahoo then skipping on app-required Proton & Tutanota.