I'm really not sure that you'll get that much power savings. Given the idle consumption of a drive is ~6W I'm actually surprised my 10 year old server/NAS only consumes about 110W for a system with 10 drives, 10GBase-T, 8 DIMMs, Intel 3930k and fans/some USB peripherals.
I should try testing under some load however.
If you really care about power consumption here, buy a proper NAS like a Synology.
It looks like I might be a step closer with the RPI5's pcie expansion options.