Odroid-H5 SBC with 4xM2 Slots and 10GbE
hardkernel.comPriced at €290 (for Europe)
Hardkernel offers an M.2 card with 6 SATA ports for €32. It looks like you could use three of these cards + an M.2 nvme ssd (for boot) to turn this into a massive 18 HDD storage server.