I don’t think you are missing any other decent options. I just went on the deep dive down what is out there (outside of the typically recommended Protonmail, Tuta, and occasional Posteo).
I don’t think anything other than Mailbox meets your parameters. The ones I have found which have 3/4 are brand new and not really well vetted through the industry. I was going to mention Mailfence until I started digging more in the forums here. Yikes. It is slim pickings it would seem.
CardDAV is such a killer feature. I wish it was available on more mail clients. I am right there with you on willing to give on IMAP, but CardDAV + Custom Domains just doesn’t seem to be a thing out there in the privacy respecting world of email providers. I may put in a request with Tuta to see if they could make it a feature on the roadmap or something.
Until then, I am going to try and get around to posting an issue this week requesting on Git for removing Mailbox.org from the list of recommended products given the conversation in this thread.