To sort of answer your question, I use Fastmail to host my email and contacts, and I self-host Nextcloud, and that has my calendar and tasks. I use Caldav to ha

To sort of answer your question, I use Fastmail to host my email and contacts, and I self-host Nextcloud, and that has my calendar and tasks. I use Caldav to have Fastmail see my calendar/tasks, and it is entirely transparent to me. I can also have Fastmail host my calendar/tasks, and that can use caldav as well.