What do you need in a calendar service that you can't find elsewhere?
I use the Calendar app with NextCloud[0] for a CalDAV server + calendar web interface. It's fully-featured as far as I can tell. I use its CalDAV functionality to sync to my Android phone using DAVx^5[1], which integrates the calendars natively so that you can use whatever calendar app you want on your phone (I use Etar[2]).