The safest method, if your /home has enough space, is to use it instead of /var for (some) Flatpak installs. You can force any Flatpak install to go to /home by adding --user to the command.

If you look at the output of flatpak list it will tell you which package is installed in user home dir and which in system (/var). You can also show the size of each package with flatpak list --columns=name,application,version,size,installation.

I don’t think you can move installed apps directly between system/user like Steam can (Flatpak is REALLY overdue for a good package manager) but you can uninstall apps from system, then run flatpak remove --unused, then install them again with --user.

Please note that apps installed with --user are only seen by the user that installed them. Also you’ll have to cleanup separately for system and user(s) in the future (flatpak remove --unused for system, then flatpak remove --unused --user for each user).