Transform Nuc running ROON in a NAS somehow

I’m using a NUC8 i5 with 16GB ram and 1tb M2 & 2tb ssd as music server. Mainly running ROON and Plex. Usage is 95% Roon. I would consider myself a music enthousiast and collector.
I have 2 * 6TB external harddrives, one for music and one for backup. Each one has a 1on1 spare so 4 external 6TB Seagate Expansion Disks in total. And now I’m running out of disk storage.
So my music collection is exceeding 6TB.
I was allready collection the same external harddrives, so I’m able to buy a Synology one time, and get those disks out of theyre cages and put them in a 4 disk nas enclosure.
But I’m not there yet, and I think there is an issue with my logics. As I’m unable to empty 2 of the 4 disks, I probably won’t be able to build a RAID5 with the same 4 disks in a Synology. So I’l need an extra 12TB external harddrive. That way I also have a backup of my music collection off the NAS.
But I don’t have the budget for both.

Is there a way to build a reliable storage pool on my nuc using windows 11? So if I’m able to build a storage pool with these four external disks, using one as a spare, I gain an entire disk, and solve my storage issues.

Then I have some time to get the budget for the large external harddrive and de synology nas enclosure. Because I’ll need both to make the transition to a NAS.

The only thing then is that my music server, and storage server are one and the same device. Instead of the initial plan: tranfer everything to the nas, install plex on nas, install rock on NUC.
But theat probably ain’t an issue as its a i5 with 16GB of rams only to be a music and data server.

What look like interesting options here?

Thanks in advance!