Here is an update. In another thread it was discussed how the /var/lib/flatpak/appstream/flathub/aarch64 directory can get clogged up with (literally) thousands of unneeded files (in hundreds of directories of which the names consist of a very long hex number and which start with a dot).
I renamed the flathub directory, copied the ‘active’ directory that lives there, as well as the only directory with a long hex number name that is not preceded with a dot to a new, empty ‘aarch64’ directory. Then I waited a couple of days to see what would happen. And when no errors occured I deleted the original flathub directory.
This gave me four or five extra gigabytes of space(!).
I thought I’d mention this here in order to gather this sort of info into one thread.
Use at your own risk, of course. I know others have also deleted all this dross without a problem, but still.