So I looked this up now (have not tried it myself, though).
So looks like gnome-calendar should support CalDAV calendars since some time now?
https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_reques...
However, according to this issue calendars work once added but the only GUI-way to add them right now still seems to be Evolution, after which they will show up in gnome-calendar as well (also some workarounds here):
https://gitlab.gnome.org/GNOME/gnome-contacts/-/issues/70
There's a PR for proper support here but maintainers seem MIA:
https://gitlab.gnome.org/GNOME/gnome-online-accounts/-/merge...
Had some FUBAR situations where vdirsyncer tried to fix "something" (which is necessary from time to time since davmail does only rudimentary checks of events) and got im a situation where I send confirmations for 5 year old events to a few dozend people
I run a calendar server[0] that is my main calendar repository. And I use vdirsyncer[1] to sync to my laptop to use khal[3]. On the Android side I sync with davx5[4].
This allows me to subscribe to webcal calendars, which outlook365 offers, and syncs them to my two main devices.
Not perfect because that means I can't edit my outlook calendar from within this system ( and a reminder why open standards and interoperability are important ), but it means that my private calendars and addressbooks are self hosted.
You are right though, that takes effort. And I want to snark at microsoft for not supporting caldav, but then I am waiting for gnome-calendar to support it as well. It doesn't seme to and that feels a bit strange to me.
[0] https://radicale.org/ [1] https://github.com/pimutils/vdirsyncer [2] https://github.com/pimutils/khal [3] https://github.com/pimutils/khal [4] https://www.davx5.com/