For the last 20+ years I have always built my own home NAS. 6 to 8 drives. ZFS. Initially OpenSolaris, then FreeBSD, and nowadays Debian. I would hate to use a proprietary solution like Synology. I'm generally very happy with my builds, hardware-wise and software-wise. Surprisingly the most annoying thing of a custom build is that there aren't many choices of decent compact 8-drive NAS chassis (it's a small market after all). By compact, I mean micro-ATX or smaller. Because there is no standard placement of SATA ports on a motherboard, sometimes depending on the combination of motherboard and chassis, it's physically impossible to plug in all cables in the ports. Even when using a combination of straight and right-angle cables. Currently I'm using https://www.alibaba.com/product-detail/MATX-ITX-mainboard-8-... (many Chinese companies sell similar clones) It's not as compact as an 8-drive Synology NAS chassis but it's compact enough for me.

Yes, a standard Linux computer is the way to go. I would argue most people do not need complicated redundant drive setups. A regular computer with a standard SSD works fine. Probably good to start out with a new one though and not reuse an old one from an old computer.