You've described the xdg-desktop-portal. Gtk apps just need to switch to GtkFileChooserNative and they get that for free. It does an FD pass of a FUSE fd and the app never gets file-system access.
It also has the benefit of using the KDE file-chooser on KDE, Windows file-chooser on Windows, mac, etc...
With the current situation, calling the programs sandboxed is completely misleading.