A NAS is just a small desktop computer. If you have a motherboard/CPU/ram/Ethernet/case and a lot of SSDs/HDDs you are good to go.
Just don’t bother to buy something marketed as NAS. It’s expensive and less modular than any desktop PC.
Just my opinion.
The moment people realize a NAS is just a small factor desktop with a lot of space with good Ethernet speed, their eyes widen up the same way people realized the cloud is just someone else’s computer.