So you can enable it and then run:

sudo fwupdmgr get-updates

And you will get a list of what would be updated with the testing repo, and then you can disable it again if it doesn’t return a result that one can work with.

You could try enabling the lvfs test-repo

sudo fwupdmgr enable-remote lvfs-testing

And the run

sudo fwupdmgr update

Ok, so this is a ”how many colors of the rainbow there are”

If you go balls to the wall, all out retardation: Nextcloud, the CalDAV is basically just a bonus, I’ve replaced google.com at home with Nextcloud

Some middle ground: SoGo, sogo.nu

If you just want to solve this specific problem: Radicale, https://radicale.org and Baikal, https://sabre.io/baikal/