

The Google account is still there, I’ve just hidden each of those calendars. New events seem to default to whichever calendar you used last (citation needed) so once you’ve created an event under the NextCloud account, it should default to that. The app I use (Business Calendar Pro) doesn’t even give the option if a particular calendar is disabled.
As far as DAVx5, the default is to only sync every 4 hours. Phone to NC is instant (once DAVx sees the change), but NC to Phone has a delay. However!..
I just set up the ntfy app to create instant two way sync both ways. It’s pretty slick. Dig around in the DAVx5 documentation about how that works. You install the DAV Push app on the NC server, ntfy on your phone, enable UnifiedPush in the DAVx settings, do a little dance, and then NC notifies DAVx every time there’s a change.
Edit: you can also delete calendars from Google Calendar settings on the web, but this is obviously permanent. I think you can also totally remove Calendar from your account.
Edit2: You can’t remove calendar from your account, only delete individual calendars.
I use gmail. You just have to set up an “app” password. I always have to search for how to do that, but once you have an app password you’re off and running.
I also just started hosting my own nfty and have been moving as much as possible to that. So far I’ve replaced two email notifications with push notifications, which is nice.