I've been using Baikal for a few years now and it's very solid, although it's just for myself and I don't have all of OP's requirements. For my desktop I just u

I've been using Baikal for a few years now and it's very solid, although it's just for myself and I don't have all of OP's requirements. For my desktop I just use Thunderbird's built in calendar to connect, and on my phone I use DAVx5 and Fossify Calendar from F-Droid. It's forked from Simple Mobile Tools's Calendar app, but unlike the name it's actually the most fully-featured calendar app I've seen. It's "simple" in the way that it just presents you with every option instead of forcing you do dig through half a dozen menus and dialogs to do what you want to do, which is very nice.