I went with Posteo and have been happy with it. The only meaningful advantage of Tutanova and Proton that I'm aware of is the encryption, but that only works if the person you're communicating with also uses their service, so I figure what's the point? I've also heard of various problems people have had with with both of them. And, at least at the time I checked, you had to use a browser or electron interface to access them though their own UI. With Posteo I added the account to Thunderbird and I access it through SMTP via TB's mechanisms (e.g. auto-checking every X minutes and popping up a system notification when there's a new email) just like with my other email account.