For now, it's Debian 12 with KDE Plasma. But I'm really interested in Immutable Systems. I like OpenSuse Kapla, but the KDE Integration is still in alpha. There are still a few shortcomings with the only flatpak approach, like the fact that the Steam Flatpak can't provide smooth wireless controller support because of lacking permissions.
I've found success installing Steam and other stuff using distrobox on openSUSE Kalpa. The initial setup isn't as easy as installing a flatpak, but after a quick distrobox-export it's totally seamless.