Steam creates its pressure-vessel containers using namespaces, and there is a Steam flatpak, which I think was made possible by some work a few years ago specifically for the purpose of nesting. I don't know if that work applied to flatpak, bubblewrap, or both. It might be worth investigating.

https://gitlab.steamos.cloud/steamrt/steam-runtime-tools/-/t...

https://github.com/flathub/com.valvesoftware.Steam