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.
I’ve been using Posteo for a few months now. Have 2 dedicated aliases setup (more than 2 is €0.25 per) with filters effectively making two separate inboxes for them.
It does well, web app sometimes fails over to Deutsch at random, the login cookies are very short lived (good to prevent access from local intrusions), and supports 2FA.