UGREEN isn't really build your own. The hardware is similar to Synology (but I think slightly better made, and definitely with much higher specs). But unlike Synology it's easy to install your own OS. I used Fedora, but a lot of people are using TrueNAS which is almost as turn-key as the Synology software.

For reference I own 2 x Synology, 1 x UGREEN and 1 x QNAP; and will likely replace the other machines with more UGREEN in future as long as they don't do anything stupid.