My solution looks very similar:
SLUG1 (Openslug 3.1) hosts the music files in folder /opt/mp3.
SLUG2 (SugOS/BE, installed on a 2 GB USB stick with no disk) hosts firefly.
SLUG1 serves as NFS server, SLUG2 as NFS client.
SLUG2 has mounted the /opt/mp3 folder under /mnt/mp3. FireFly reads without any problem from this mounted drive.
Has been totally stable for months now!
SLUG 2 is always on (hosts a webserver as well), SLUG1 is shut down from time to time … (if there is not much to backup and if there is nobody home who wants to listen to music …)
I do not use SMB (Samba) on my slugs because it did not interoperate well with our XP machines (made the Windows File Explorer freeze all the time).