If you’re talking about Davx5, it actually works great for calendar/contact sync and is (mostly?) OSS. At least, I know I can install it from f-droid…

I’ve been happy with mailbox.org for years, too, yeah.

My only complaint is their WebDav experience is…just not very good. It’s probably that I wanted a sync client and not a webdav client. But it wasn’t quite standard (iirc) so getting everything configured was a pain…