I too recently looked for a dumbphone, open source and privacy-oriented, with a way to communicate using an open standard messenger protocol (XMPP, Matrix) and CalDAV support – so basically either a web browser or a good app store, with decent battery life. I could not find anything, at all, and gave up.

Imo the variety is lousy. There are countless models to choose from, but they all pretty much work the same, come with vendor lock-in, uninstallable apps, integrated Google services and all of that.