There’s also U.2 and I’m going to be reworking the rear-mounted bracket 45Drives published as an STL to support both 15mm drives and hopefully six drives, then connecting 6x 6.4TB NVME with U.2 to my system through MCIO ports on the H13SSL-N.
Have the drives in hand. Have the first HL15 showing up tomorrow so will report back soon!
I suspect if you don’t mind the U.3 not being a proper backplane setup you could do similar — just an exercise in how much I can cram into the case above and around the mainboard.