NAS with 10gb ethernet, probably via PCIe slots. Custom probably makes sense if you're making lots of the things, otherwise only as a passion project.
NVME drives seem to be cheaper than I remember - you might be holding a depreciating asset.
Cheap Dell server, dual socket, with lots of PCI slots. Or, dual Xeon motherboard ($200 on eBay) with low power Xeons. Two CPUs and sockets mean more lanes. Add that with $30 pcie boards and you have a decently cheap NAS.
NAS with 10gb ethernet, probably via PCIe slots. Custom probably makes sense if you're making lots of the things, otherwise only as a passion project.
NVME drives seem to be cheaper than I remember - you might be holding a depreciating asset.