Have you checked out the recently added distrobox-assemble feature?
You can define the container (or multiple containers) and what software to add (and very soon what apps and bins to export to the host) in a single config file declaratively.
It’s the killer app for me to live alongside my Silverblue install!
I can’t answer your question but Thunderbird just officially took over the flatpak on flathub.
Maybe a good alternative?