You are going to have a problem with lack of pcie lanes on that x10sri. If you want to use old xeons then get rid of the x10sri and pickup a x10dri and an extra e5 cpu, about $160 on ebay. That get you 3 gen3 x16 and 3 gen3 x8. This would let you setup 2 hyper m.2 with 8 2tb nvme in a 2 vdev raidz2. 11-12 TiB of fast storage for a grand. You would still have slots for 10gig nics, 12Gb SAS controller etc. You could also use the 10 SATA ports for your slow storage.