An interesting read, and I have to say I do lean more towards building your own. I did buy a bare-bones NAS by D-Link many years ago because you could add your own hard drives, but what I discovered after a few years was there were no more software updates, and it is not easy to upgrade later on either.
Today I’m running a bare-bones Intel NUC box with two external laptop hard drives (powered through USB), and Open Media Vault NAS software on it. I get ongoing software updates and can upgrade RAM and drives etc. That said, its throughput is not that great as the drives are connected via USB cables. It boots from a SATA connected SSD drive. It also has Docker running on it with a few applications hosted in Docker containers, so it is quite versatile.
Knowing what I know now, I would rather have bought something that would allow proper SATA connectors to the hard drives for way better throughput speed.
See https://www.xda-developers.com/building-vs-pre-built-nas/