Huh, I never considered it from the iMessage standpoint. Isn’t that (currently) just a messaging protocol? I didn’t think it manages your contacts. I thought Apple just used CardDAV, which supports contact images. It was their excuse for why they couldn’t end to end encrypt that data, with ADP.
Tutanota. I used to use Proton, but they don’t encrypt folder names, which is a deal breaker. Tutanota does, and they’re also a privacy respecting, reputable, decent service.