I have used many of the appliance operating systems in existence, including TrueNAS over the years, hence why I’ve come to the conclusion I’ve come to. I used to evangelise them, particularly FreeNAS and pfsense. I used them for years and I was quite happy with them.
But eventually you run into a bug with their (bolted on) interface and you have to dig inside the guts anyway. Except now it’s 10 times more difficult than just using the base OS, Debian FreeBSD or whatever, because of the extensive changes made by the appliance OS vendor…
If you are looking to replicate the exact same thing(why would a NAS need a web ui), of course there is considerable work ahead of you.
I believe you are overstating how difficult it is to configure a FreeBSD box as a NAS. Configuring samba for example is a breeze.
The questions you are asking are good questions that are answered by the manual. Keeping FresBSD updated requires the use of two commands, both well documented in the handbook, freebsd-update and pkg.
With a FreeBSD or Linux machine, even step one requires considerable thought. Are there web UI packages that I can use? Which one do I pick? Where do I install it from? How do I ensure that it runs on boot? Do I have to mess with the network configuration to ensure that http://mynas.local is accessible? How do I configure SMB? What's the deal with security updates? And so on, dozens of times over.
It's great if you're already in the depths of sysadmindom and know what you're doing, but man, I just want to put my files on some LAN drives and call it a day.