I just ran into this exact same problem, and have found a solution in case anyone is still experiencing trouble with this.
Background on my situation:
I just moved from a Mac Mini to a Nucleus. I installed an internal 2TB SSD in the Nucleus to hold my iTunes library. After formatting the internal SSD as directed in the Nucleus manual, I was able to mount it via my network from a separate Mac laptop using the SMB server path method. I then manually copied my iTunes library into the Storage/InternalStorage folder on the Nucleus via the network (this is a little confusing because it’s hard to tell which folders on the Nucleus are on the Nucleus’s small built-in SSD and which are on the big SSD that I’d installed myself; reading the Nucleus documentation I deduced the Storage/InternalStorage indicates the big SSD). Working from the Mac laptop, as long as I mount the Nucleus via the SMB server path first, I can then option-click the iTunes icon to choose the iTunes library on the Nucleus before opening iTunes on the laptop, and then my playlist edits, new music, etc. are saved to the Nucleus SSD. I set Roon to watch the iTunes library on the Nucleus internal SSD, and everything, including new changes made via iTunes, shows up perfectly in Roon.
To back up the iTunes music library now on the Nucleus (or whatever form of music files you have stored on a Nucleus internal SSD), I wanted to be able to now copy the files back over the network from the Nucleus to an external backup drive connected to my Mac laptop. And, importantly, after doing this the first time, I wanted to be able to do subsequent backups this way but only incrementally – only copying new changes, not the whole library every time. For this, manually moving certain files is technically possible, but not at all feasible or desirable.
The three candidates for a backup utility that would could do this, that I’m familiar with for the Mac, were SuperDuper!, ChronoSync, and Carbon Copy Cloner. Maybe I am missing some things here, but based on my brief research and testing: The problem with SuperDuper! is that over a network, it seems to only work the other direction – copying to a network volume, instead of copying from a network volume – and only to a Mac disk image that you create on the network volume. ChronoSync seems to be able to copy from a network volume, but the problem here is that it seems to require the network volume to be formatted for Mac and to have a ChronoSync agent installed on the remote computer; neither of these is easily doable or even possible with the Nucleus, that I can tell.
Carbon Copy Cloner, however, worked just fine without any tweaking. With the Nucleus mounted on the Mac laptop via SMB, Carbon Copy Cloner easily allowed me to select the Storage/InternalStorage/iTunes folder on the remote Nucleus SSD volume as the source for backing up, and then allowed me to choose the destination as a folder on an external drive attached to the Mac laptop. I’ve successfully completed the first full backup this way over the network, and now future backups run with the same Carbon Copy Cloner task will simply be quick incremental backups.
I hope this helps!