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.
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.