Does anyone know why there's no desire to see a working standard (protocol) for calendar/tasks?
It's clear that CalDAV doesn't function consistently across devices (and does seem to be dying as a standard). If you work across different devices/OS it's virtually impossible to get things set up seamlessly. Companies and developers of task apps seem happy to create silos and not look at interoperability.
If you want to self-host, it's too hard to do this and you really are limited to a tiny number of options.
Guess I was always using the right combination of apps. Never had any problems with CalDAV and CardDAV. Except for frustration at outright missing support.
Nextcloud as the server and DAVx⁵ with Fossify Calendar as the client on my phone. On my laptop Kontact, but I haven't looked at that in ages, so it could be in shambles for all I know. I think I've also used Thunderbird with some plugin.
And my Fritzbox router uses CardDAV to populate the phonebook of all connected phones.
You are probably the exception rather than the rule. Nextcloud, in my experience, is the only one that seems to work fairly well. Just look at the Issues on Github for apps that try to use CalDAV.