Imho for a single drive nas solution a pi is way too much computer. and far simpler and cheaper solutions exist off the shelf
something like https://www.gearbest.com/wireless-route ... id=1433363
which you can flash with openwrt if you dont like the out of the box firmware.
If looking to add to your proposed pi nas with more disks as you need them then the route i took was to buy a nice multiport usb hub into which i have connected 6 x usb hdd enclosures (4 x usb spare). power for the disks(12v) and the usb hub (5v) is provided by an atx psu, its power is switched by the pi and a tasmota/sonoff basic switch. to stop power back feeding from the usb hub i have cut the 5v rail from the hub usb/out. it was a minor pita connecting 12v plugs to leads and the 12v feds from the atx psu
the pi has its own dedicated psu, so generally the atx psu and disks are not powered up.
when disks are needed, the pi starts the atx psu and boots up the disks before mounting. when done the disks are dismounted and the atx psu is shut down. the plan was to schedule disk startups but i simply kick off the disksOn and disksOff scrips which simply contain the
the power on command followed by 6x mount commands, and off is 6 x umounts followed by the poweroff..
power on and power off are a bit more involved as im using sonoff/tasmota a/c switches controlled by wifi and nodered on the pi, but a gpio triggered relay would work just as well ..
My own quandry is it box or not, currently the disks are all enclosed in their own boxes but the psus/pi /hub and a lot of wires are tucked away on the top shelf under the stairs.. the benefit of boxing vs the hassle