You didn’t say what OS you were going to put on the NUC.
If you’re going to use ROCK, then that’s the only thing that can be on the M.2. You don’t need anything larger than 128GB for that.
The NUC12WShi7 can only use a SATA SSD as the 2nd drive. There’s no operational difference between an M.2 drive and a SATA SSD.
Many people use an external USB drive for their music files. The only drive that needs to an SSD drive is the drive that has the Roon app or the ROCK OS. Music files are just fine on an external USB HDD. If one uses an external drive, then to add music the drive can be temporarily moved to another PC for copying, rather than needing to do copys over the network.
Well, I am not sure it supports two m.2 slots for storage, I think one is used for wireless. The OS drive is only for the OS and you cannot use it for anything else including music. So you will need a second drive if you want to have music stored internally which will be the SSD connection. Although, you can always use an external USB drive instead, and would suggest you do so for various reasons.
Also, M.2 is wasted on storage, so is an SSD. An external HDD via USB is already faster than is needed for playback. Any additional speed is not useful.
There are 3 PCIE, Key M (SSD), Key B (SSD) and a Key E which is an M.2 used for wireless. Note that I drew a mark between the two for the wireless.
These are physically different connectors, (i.e. the 22x80, 22x42 and 22x30). It might be possible to remove the internal Wifi card, and buy adapters that would let you use the Key E for an SSD. I believe that is talked about here in a tinkering blog (Replacing a Wifi card with an PCI-e SSD).
The blog also has a brief run down of the different cards with pics to see the diff shape/sizes and their general usage. (storage vs wifi vs bluetooth)