To be fair if i had an SSD i may well be inclined to do so, but its the NAS im more concerned with. I really don't want that running the disks 24/7, nothing to do with electricity.
That's generally not how it works.
Spinning disks up and down == difficult.
Keeping disks already spinning == easy.
For electricity conservation purposes, and to prevent head crashes on laptops and other portable devices (that might get dropped at any moment), it makes sense to spin them down when idle. For longevity purposes, the best thing is to keep them spinning
always, unless they'll be unused for a LONG time (cold storage).
I would imagine that unless the mini goes to sleep then the NAS wont either
I'm not sure how your NAS works, but this is also very unlikely to be true. The NAS should be able to park disks regardless of the state of any connected network device.