Every KDE app I try (and the Plasma desktop) seems so good on paper, and they promise me the world! Then, wen I actually try them out, they always end up crashing or doing something weird. Like I cannot stand GIMP, so I've tried using Krita, but I don't think I've ever managed to finish something in it before it crashes. It's the same with Kdenlive.

Damn shame.

That's pretty weird; both Gimp and Krita (very different tools) were rock-solid to me. Speaking of KDEnlive, I experienced a few anomalies and crashes using the version from my distro; I switched to an AppImage version, and with it, everything Just Works™.

I suspect your crashes may also be related to dependencies, not some deficiencies of the application itself. Try a different build / AppImage / Flatpak, and see if you encounter the same problems.

I don’t use any KDE apps, but the Plasma desktop has been absolutely rock solid and super performant for me.

I do think that the idea that each toolkit has its own native app for each thing you might want to do with a computer is a recipe for a forest of half-maintained nearly-good apps. A lot of the KDE and GNOME app suites feel like checking boxes.