How much TB can the internal SSD be and will the SSD function as a network storage device, i.e. can I approach the SSD and its files from any device on the network (iMac, iPhone, etc.) in order (i) to use - from time to time - alternative software tools to edit the library, (ii) load new music files to the database, or (iii) to have JRiver or Plex also use the same music database from the Nucleus (which today I do from a Synology NAS).
You can get 8TB SSDs these days that will fit in the Nucleus. Since this storage is for music files, it is “read-mostly”, and the cheaper QVO technology is perfectly adequate for this purpose.
And, yes, the internal storage is accessible over the network, in fact this is how you can transfer music files onto it.