I've been using a Synology DS415+ for 7yrs and love it. I use WD Red drives in a RAID5 configuration because Synology's newer/flexible RAID format wasn't an option then. This gives me redundancy that allows me to replace a damaged drive and replace/rebuild in a couple of clicks for little cost. All drives go bad eventually so this is a good protection, but it DOESN'T replace a separate backup!
I have about 4-5TB of music which is ~40% of my capacity. That's about 4K albums, probably 1/4 in large hi-rez, and about 1K full concert soundboard recordings. I can't imagine ever needing more storage even though I have plenty of stuff not available on any streaming services. Now that drives are cheaper, if starting out, I'd go with large size just to be safe, but with the newer RAID option, you can mix sizes between drives.
If you have the luxury of putting your NAS wherever your router is (i.e. outside the listening room) mechanical drive noise isn't an issue. SSDs cost WAY more, but if in the listening it's worth considering.
I use the common method of an ethernet switch between router and NAS, and wire ethernet to listening room to an OpticalRendu. See my system page for details, sbank's Austin City Within Limits Cheers,
Spencer