Are you trying to solve one problem, but then find the next?

If you solve the problem of the media selection (which looks like some form of database replication), then what happens if both parents select media they don’t have at the same / similar time - you’ve stated elsewhere that this is the bottleneck.

I think it’s going to be much simpler to just replicate the media and let them work locally.

I don’t know what your script with rsync is doing, but syncthing can limit bandwidth and have exclusion patterns, so perhaps one set of parents don’t want anything from GenreA and you could exclude that.

Similarly if both parents lile GenreC and you don’t, then just sync that between their systems and save your storage.