I have a 4 bay Synology and an HP G2 800 i7-6700, plus a POE switch, couple of cameras, omada WAP. The software load is mostly Jellyfin and Syncthing plus a BOINC LXC that pegs one core. Power consumption generally sits at around 55-65W (my APC UPS has a power readout) from memory the idle was pretty low - I think 24W although that might have been the previous 2 bay NAS.

I think your plan of a NUC and a NAS (I’d stick to 2 bay) is a good compromise for low power and easy of setup/management. I have my NAS configured to keep the rust spinning - I’m sure I’d save a bit of power by letting it spin down, but the delay when I starting some media in Jellyfin was annoying, and I suspect the disks will live longer moving anyway.