@rfc2822 Thank you for the very immediate reply on this Saturday evening.
I just tried aCalendar+ (not open source but they seem support worthy). This is hilarious. For task support they rely on the OpenTasks app, which I’ve tested before and really disliked. The task management in aCal+ itself is very rudimentary, and basically only useful for the calendar integration. So I need another app for task management (I currently use Tasks.org), which I’d be fine with except for the fact that I now have 3 apps installed to do one job (4 if you count DAVx5). :')
It turns out to be quite the headache trying to scrub off the big Goo.