With the Asrock Rack C3758D4I-4L it looks like any 10gbe card I use will take at least 4 lanes and according to it's manual, it will drop the SATA capacity to 9, but that is in 6gbs. I'm wondering if I can I keep the onboard 5 SATA at 6gbs, and put the 8 SAS at 3gbs so I can squeeze in a 10gbe card and have at least 11 SATA connections. The manual doesn't mention this so I have to assume no, but that might not be definitive.
The board looks really good, but since that Atom only has 16 lanes... not so good. The only other option I see would be to go with an embedded EPYC and that literally doubles the cost for which would be essentially for keeping only 2 drives off of USB.