For future reference, if anyone cares 
, I ended up doing the following
- Create iSCSI drive on my NAS
- This keeps the advantage of having my music in a central, backed up location
- No concurrent access from multiple devices though, so my Plex server no longer has access to the music
- Not for the faint of heart, but definitely doable
- Copy music from the SMB folder to the iSCSI drive
- Edit the rekordbox collection XML, replacing ‘/Z:/’ with ‘/D:/’ to refer to the iSCSI drive
- Reimport the Rekordbox collection
Not ideal for me, but I rarely access my music collection anymore these days, so good enough. And as long as I have enough space on my NAS, I’ll keep my collection duplicated there between the SMB folder and the iSCSI drive anyway, probably.