I tried Kontact (integrating Korganizer and Kaddressbook into one application), and it was almost perfect, except for being too buggy to use. Evolution (what I'm currently on) is so slow that using it is a pain. I would like an upgrade. Here are my requirements beyond what is standard for applications in this category:

* It must be an application that runs on my computer.

* I have no preference for whether it is a single application or two separate applications.

* Qt is preferred over GTK, which is preferred over Electron.

* All applications that handle sensitive personal data must be open source. Therefore, so must this.

* It must be able to synchronize with an on-site server, which I strongly prefer to be Nextcloud (so CalDAV/CardDAV protocols)

* It must be able to read and write my cloud database offline and have changes pushed and pulled in the background when I'm online.

Is there anything else like this, or am I stuck with Evolution?

Thunderbird.

+1 for Thunderbird. I use it with Google calendar.

1 members found this post helpful.

I just set up Thunderbird, but I am having two issues with it:

1. I cannot show my calendar. Opening the properties shows that it is turned off. If I turn it on, nothing happens. Opening the properties again still shows it as off.

2. I cannot figure out how to make it synchronize contacts.

Left click the icon on the very top right to view the calendar.

I found the calendar icon in the very top right. After clicking on that, there is a calendar view, with a zoomed out month calendar in the top left to show me where I am. Below it, there is the place where my calendars are listed. In this case, only one because I only have one. Next to it is a checkbox that is supposed to dictate whether or not events from that calendar appear. The check box is not clickable and the calendar is grayed out. I can right click on it and hit properties, and this is where the checkbox I'm talking about is.

Another vote for Thunderbird. I've been using it for over 20 years... all the way back to my MS Windoze daze. I currently use it in Slackware Linux along with the Calendar extension. I've stated elsewhere that if T-bird ever disappears from the world, my computer-ing/internet-ing days would be over with.

Here's a screenie of my highly customized T-bird:

https://i.imgur.com/49gbTbn.png

I use "remind" from "roaring penguin"
(that always makes it sound like a proprietary thing but remind is such a generic word that it's easier to search with the author added) for reminders, todo lists, calendaring, etc.
Not sure if it syncs like that, though, it uses plaintext files...

...and I keep my contacts separate, so my reply here is probably absolutely unhelpful to you.
(But it gives more exposure to remind, and it's a great piece of software :P, and who knows, maybe one day you'll get interested in it. )

Oh, it seems there are third party programs/tools that allow syncing in that CalDAV format after all.

Last time I looked, Thunderbird could not see third party (not part of your Email account) CalDAV calendars/address books without an addon.
On Linux, Evolution is the only one that does this OOTB. To my knowledge.

1 members found this post helpful.

True (about TB) -- an add-on is required to link to the Google calendar I use.