OK here is a recipe to consider.
One NVMe M.2 consumer drive is 3-4X faster than any SATA3 single drive.
So you could simultaneously image 3 or 4 drives at a time at little cost with modern hardware..
1
M.2 NVMe 512GB=~$200USD
4 ESATA hard drive docking ports=~$100USD (Number of docks=number of free SATA3 ports on your mainboard.)
1
NVME paddle card for PCIe 3.0 Slot=$10
You might end up needing more ESATA docks depending on speeds.
Will need to externally
breakout the SATA to ESATA.
You will need a PCIe 3.0 slot for the NVMe, so newer hardware required.
Most of the external Hard Drive docks have combination USB and ESATA.
They are complete garbage for saturating 550mb/sec SATA3 (At least all that I tried)
http://www.highpoint-tech.com/USA_new/series_RS5322-overview.htm
You also need to check your motherboard to see if it allows for
hotswap SATA drives in bios. Most server boards do.
Otherwise you might need to powerdown after each image writing cycle to disconnect drives.
This comes back to why most docks use USB. The disk disconnect method is standard.
FreeBSD can boot off the NVMe so you can use all your SATA3 ports for docks.