Commenting here since all the Github issues are locked...
Just performed the nextcloud upgrade that's currently sitting on master... Everything went without a hitch. Great work!
I spent some more time looking at DAViCal since NextCloud still makes me uncomforatble... There seem to be issues like https://gitlab.com/davical-project/davical/issues/84 that imply its interop isn't as wide as NextCloud.
So.... NextCloud does seem to be the most appropriate. For now.