I'd be curious to know what the actual NAS performance is like (IOPS, latency, etc).

Looked into building a DIY NAS a few years ago, but decided to buy (a Synology DS1618+) instead. I'm sure I could've saved a few hundred rolling my own, but having purpose build hardware + commercially supported software is worth the cost if it's data that you care about (versus something like storage for 'linux iso files').

I actually had an issue a few months back where my storage pool suddenly degraded and went read only. Was able to send a full diagnostics/log dump to Synology, and their support engineers took over to diagnose the issue. If I rolled my own, I'd be the one spending hours either figuring it out myself while stressing about losing data, and/or rebuilding entirely from my offsite backup.

I've looked into using SBCs for a DIY a lot, if you're fine with no hotswap and 4 disk then it's doable on the cheap. Since I wanted more than 4 drives and hotswap I gave up and ended up looking for a dedicated NAS. The issues with SBCs are usually the adapters you end up using have SATA controllers limited to 4 (or 6 for ASM1166) drives or use SATA port multiplers which reduces performance. For home uses, I don't think this matters that much tbh. Adding hotswap is costly due to backplanes and when you have more than 4 disks you run into annoying things like needing more power so you need a real PSU for your SBC. The best solution I though of for this was PicoPSU to keep the size down or FlexATX but the latter adds up in cost. After seeing the size and all these costs add up I just gave up.

I put together a DIY solution, but I like my synology better.