

The space inefficiency is definitely there.
I find that clients, such as Jellyfin, Moonlight and Signal, works just fine as flatpaks but with those three apps my /var/lib/flatpak/ lands on 6.4GB.
When I temporarily had Discord installed it grew to 6.7GB, so the inefficiency is frontloaded and lessens the more of them you use.
Yeah, I’ve written some custom css to get some better wrapping of libraries and such.
There’s also the community themes worth looking into.
https://jellyfin.org/docs/general/clients/css-customization/#community-themes