I am switching back to Android after nearly 10 years and:
- no native CalDAV support
- third party open-source DavX offers that, although it's hacky and the push support is even more experimental
- but, most importnatly, your Google account's calendar (which you need for Play Store) is still a default and any 3rd party app adding an event to your calendar will adds it to the Google one. You don't get to change the default calendar, you don't get to disable it!
- you ned to give Gemini full access to your Google Calendar (again) to get it to save your appointments or reminders. You can't use it with non-Google calendar.
None of these were an issue on iPhone, I was able to use my NextCloud instance to host all my events privately and securely, including with Siri.
I am actually super frustrated and find it odd that this is even legal in EU, because it definitely looks like protectionism.