Debian with Flatpak and a Distrobox container running Arch is pretty good if you want a stable desktop with rolling packages.

The forum post says that plasma-integration will conflict with qt5ct or qt6ct, so you could try removing those packages and see if that works.

I was having the same issue with discover when I was trying LXQT a few weeks ago and I wasn’t able to find a fix. This post made me look into it more and according to this reddit post, Discover uses the Kirigami UI framework, this led me to this forum post which says that you need to install the ‘plasma-integration’ package to make QT themes work with Kirigami based applications.