Haven’t tried it, since I only sync with my PC and cloud server. But I just checked up and Librera has an option to save progress and profiles to a custom directory, so I guess that would be the way.
These are the ones I actively use with syncthing
Also most routers with an USB can also handle a printer
I have a similar setup. I have an old laptop with an SSD that puts files on my NAS. The laptop has arr setup on containers, the laptop itself is running debian, mounts from Samba NAS via fstab (be sure to have writing permissions) has been working that way for several years.
Only direct plays, no transcoding (core 2 procesor)