I use decsyncCC - which can use a folder as a form of DAV - combined with syncthing across devices. This requires no server and works well, the only problem is idk of any good decsyncCC folder-structure parsers that can be accessed as a local DAV service from the desktop.
Here's a github topic about Caldav. You might find something in at least one of the repos. https://github.com/topics/caldav
I'm not rrleally sure how calash clients work, but maybe you can sync them manually with something like Syncthing.
Why is Nextcloud not what you're looking for? Sounds perfect for what you're after.
You need to self-host it.
Oh ok you should probably clarify that in the OP since everyone is giving you recommendations of self hosted software. It's usually assumed that's what someone's after when they are asking for a FOSS service.
There are some places that have one click (prepackaged ) Nextcloud installations. I set up one on Vultr in just a few minutes. If you’re savvy enough to even know that you want FOSS, which I applaud heartily, then you can handle a Nextcloud installation.
Lots of options to choose from here: https://github.com/awesome-selfhosted/awesome-selfhosted#calendar--contacts---caldav-or-carddav-servers
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