Most NAS on the market are built around an ARM SBC with similar or better compute specs to the RPi and run Linux (often Debian). Honestly I think the Synology NAS even has a bigger and more helpful community for running these kinds of applications than the RPi does. There are also some more tinkerer-oriented NAS products such as the Kobol Helios machines that even offer GPIO - but also USB-C direct-attached storage and 2x gigabit (or better) ethernet interfaces, in the most recent model. When you consider the additional pain point of dealing with USB SATA controllers, I'm not convinced the RPi would ever be a good choice for a NAS or home server unless you were choosing it strictly for price reasons - NAS features like a good enclosure and drive management are unfortunately very steeply priced compared to the RPi, with commercial products often running $600 and up... although the Kobol products are very attractively priced, at the cost of very long fulfillment times since they run on preorders.

Look at HP Microservers. They're a nice package and get you ECC too. At least some NAS models can have a normal Linux distro installed as well. And if you're homebrewing a small ITX case will probably be great too. AMD has some parts that are specific to that market that look interesting.

A Synology unit can do all that, will be more reliable, and will outperform the Pi at similar price points when accounting for storage needs. Not to mention I would trust the Synology unit WAY more than a Pi.

I believe Synology NAS units support most of that. The price may not be comparable to something you could build yourself, but for those that want a turnkey solution, it's a viable option.