For my personal calendar, I have #Radicale 3.4.1 installed on a #Debian VPS. It works great, accessing it from #Thunderbird, or the "Fossify Calendar" (from #FDroid) in #Android. I can read and write events from both clients.
I was gifted an iPad, and the default Calendar app lets me have read-only access (only!) to Radicale. When it comes to writing events, that lofty privilege is reserved for iCloud Calendars only, not those riff-raff Calendars that use CalDAV.
Edit: I eventually sorted it out.
I opened a Github "Discussion" thread (for Radicale) on this:
https://github.com/Kozea/Radicale/discussions/1755
I guess the moral of this story is: Thunderbird, and Fossify Calendar, thanks for being sane! Also, kudos to #Davx5, which I also rely upon a lot in Android, for my CardDAV contacts to work properly - also stored in Radicale.
PS: #Apple is really hoity-toity. I would never choose into buying an Apple device myself, ever again. #DataSovereignty is next to impossible. #OpenSource