I mean as you can use far newer KDE applications on Debian stable via Flatpak, it may serve the same purpose contained in a separate tree without changing the core OS.
I guess distrobox+neon would work fine yes. I just wondered the state of Flatpak with the recent changes.
It will be isolated in its own directory, as I said I think distrobox.it+neon+, own home will be a far better solution of course. I keep hearing Flatpak is adding snap-like deeper features so I wondered how far it went. About the KDE 6 being unstable: I think they wanted to ship something out and for people preferring stability, 5.x LTS will be there for a long time.