You do know there are well-supported Linux SBCs for exactly this purpose available already?
As an example, I have an Odroid-HC1 (old, current is
Odroid HC4 with two vertical SATA bays).
For a backup or media storage type NAS, I would consider
Odroid M1 with at least OS on the PCIe M.2 SSD (PCIe 3.0, 2 lanes). It has one SATA connector, but you can use USB3-to-SATA for additional ones. It also consumes only about 1.3 watts when idle. Putting the OS on the PCIe means that you can use huge spinny SATA drives for backups, automatically spun down until actually accessed (although I'd also run
smartd on them, scanning them periodically about once a month, to detect data and disk deterioration hopefully in time).
Of course, if you use a large PCIe SSD for the OS, you'll have ample room for storing often-accessed data as a separate NAS volume.
Standard JMicron JMS578 etc. USB 3 to SATA bridges (that cost < USD $10) work quite well in Linux, so you can use basically any Linux SBC with gigabit ethernet or better, and one or more USB 3 host ports, to build your NAS. (For example, look at
Odroid C4 or
Odroid N2+. N2+ in particular has about twice the CPU power a Raspberry Pi 4 has, in case you need that. For a NAS, you do not. Note their low idle power consumption, on the order of two to three watts. Standby is typically less than one watt, and on a NAS, you can safely use the powersave governor.)
For a NAS box that is in heavy use, I'd consider an Intel/AMD -based box with support for three or more PCIe SSDs, and only use SATA for spinny-disk-type long-term backup storage (with the same
smartd config as for a Linux SBC NAS). However, the power consumption tends to grow by an order of magnitude.
As I build my own solutions, I am not up to date on specialized NAS distributions and such. I also do not use RPis myself, for various technical (problematic USB hardware) and nontechnical (foundation) reasons.