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 had no issues with it except for one hostel I tried to email not getting my email. The web interface isn't the best, but I access my email almost exclusively though IMAP. I've also set up my calendar and contacts though it.
I picked posteo over proton and tutamail because of the IMAP support. I didn't want to use a proprietary app.