Fair, but it’s something painless do to next time someone might be installing a new system or migrating disks. Until not too long ago I didn’t know about these kind of options, so just knowing is a good first step even without further action

A copy-on-write (CoW) filesystem like btrfs would be useful here. The “copy” would just be be shallow. The files exist in the new location but their contents are not copied, until a file in the new location is modified. And even then, not the whole contents of the file are duplicated. A “copy” like this would effectively only be a few kilo / mega bytes big rather than gigs