Why do you want ECC? (Hint: unless you're running a business database dealing with financials, you don't need it). I've run Windows server on desktop hardware since the 90's with no issues, and today's hardware is far better than what we had then.
The reason people settle on NUCs and SFF desktops is *power*. They virtually sip watts.
I don't usually recommend specifics for someone but rather ideas and ways to look at your requirements, but given your requirements (20 TB), it would be worth considering a commercial NAS, or at least a NAS enclosure running a NAS OS like UnRAID or TrueNAS.
Expansion is generally not something I'd think about for a NAS (though it can be done today). I expand my NAS once a year (swap out one drive) but I keep 3 local copies - so if it failed I can restore locally rather than from a cloud backup.
So your data lives on a NAS, and you can then either run your services there (they mostly support containers, etc these days), but I'd get a NUC or SFF to host that stuff. It makes for nice separation and gives you some flexibility.
Back to SFF and NUC - my last desktop hardware *idled* at 100 watts. It was visible on my power bill and used more power than my lights or just about any other single device other than heat or stove.
My SFF server idles at just under 20 watts and *peaks* at 80 when I'm converting videos. It currently has 8Tb of storage, but I could easily get 20 in there, it would just be expensive.
Oh, and a good NAS can spin down drives to save power when idle, which for most of us is like 90% of the time (I have an ancient NAS as redundancy that does this - it idles around 5w).