NAS devices are typically used by multiple users, that’s why they are network attached and not direct attached to a single computer. Yes, you can shut it down when not in use but it defeats many purposes of a NAS in the first place. A NAS losing power at most will lose data and at the least will require a scan at startup.

Pretty much all UPS units have a USB connector that will plug into the NAS and shutdown the system when the battery is low.

What’s your WAN connection type? I’d expect the CPU issue if you are using PPPoE, but not with static or DHCP.