Ubuntu's trust problem in 4 concrete issues - verified facts, no FUD
Strong agree. I use a derivative that blocks snaps instead of direct Kubuntu now, and it wasn’t Just because of the snaps.

I use a derivative

Without Ubuntu Pro subscription the entire Universe repository does not receive any security updates by Canonical:

canonical.com/…/ubuntu-pro-enhanced-security-and-…

You should consider switching to an entirely independent distribution that does not lock security updates behind a paywall, perhaps something based directly on Debian or Fedora.

How Ubuntu Pro delivers enhanced security and manageability for Linux Desktop users | Canonical

This week Ubuntu Pro entered general availability, giving Ubuntu users access to expanded security coverage on top of key enterprise management features. […]

Canonical
What’s a better alternative that uses apt and KDE and has relatively up-to-date packages (other than Debian testing)?
What’s wrong with Debian?
I already know about it, so there’s no need to tell me.

Fair enough.

There’s also Pop and Mint, though I don’t know if their update model differs from Ubuntu at all.

But if you’re already familiar with Debian, why not use it? It’s widely recommended for a reason, it’s hard to beat.

Pop!_OS uses COSMIC (a modified GNOME), not KDE.

Linux Mint uses Cinnamon (a modified GNOME 3) or MATE (a modified GNOME 2), not KDE.

The answer to “why not Debian” is that I try to install Debian first every time, but if it doesn’t work for whatever reason I grab Kubuntu instead of trying to troubleshoot it. 3 of the 4 desktop computers I’ve tried to install Linux on lately ended up with Kubuntu instead of Debian.

(For my personal desktop that tends to have a bleeding-edge graphics card at the time of building/installing, that’s understandable. For the other computers, for other members of my family who don’t need the latest and greatest, Debian’s failure to support several-year-old hardware – at least in the installation environment, without fiddling – was less forgivable.)

I’m sure you can install KDE on either of those.

I’m surprised Debian doesn’t Just Work for you though. I recently converted my laptop and desktop and had no issues.

Debian should be great on old hardware too. Longevity is part of their mission. The installation environent might be a bit tricky if you have really old or uncommon hardware, but in those cases I just pick the text installer, which has much fewer dependencies.