Quoting Hendrik Boom (hendrik@???):

> I'm looking for software to handle appointment calendars, contact
> lists, and todo lists.
[snippity]

> Any ideas?

Yeah. My idea is that you won't be able to satisfy your entire wishlist
even _if_ you're willing to stomach horrible, hideous, CVE-ridden
bloatware as the solution, because nothing exists that matches all of
what you said.

You're going to have to settle on whatever subset of that you most need.
Someone just mentioned Baikal -- more properly stated as Baïkal
(http://sabre.io/baikal/), one of several nifty little, modestly scoped
CalDAV/CardDAV servers. It's good. I personally think Radicale is just
a tiny bit better, but, as the Brits say, horses for courses.
https://radicale.org/

Of course, that may or may not satisfy your needs. Over to you.