@monotux actually #CalDAV is such an obvious answer I didn't think about it. Thanks for the reminder!
I have already used https://f-droid.org/en/packages/org.tasks/ in the past and was pleasantly surprised by the experience. I just need to find a web-app based client but thanks to the protocol #interoperability should be guaranteed.
However as you pointed out I have issues with my #Radicale server and calendar sharing, and it's something that's in the backlog :/






