I just swapped from proton to mailbox.org and I considered tuta heavily.

I chose mailbox over tuta because:

• tuta didn't allow third party clients like thunderbird. Given I jumped to proton from Gmail and now mailbox from proton, I wanted to decouple as many systems as possible if I had to jump again.
• mailbox, if I'm remembering correctly, had better encryption properties except for their calendar. Tuta has an encrypted calendar, and now I'm looking into a self-hosted calendar system.

I think I would still recommend tuta to like my mother or something because it's very clean and easy to set up and good enough. I'd recommend mailbox.org as a slightly harder alternative if you care about your calendar being encrypted.