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)?

@grue @woelkchen maybe try KDE Neon or SparkyLinux... 🤔

You can also use stable debian but enable surgically testing repos for KDE packages. 🤔

SparkyIinux would be your best option to avoid completely Ubuntu even as a low level basis. Otherwise Neon would be an intetesting option.
The day when Neon switches to Debian I break open a bottle of... i dont know yet but definitely something nice. 😁