You tried recording just stereo and not multiple channels?
Maybe try a test recording in QuickTime which comes with your OS.
did you try your wires and interfaces on another system? I know it sounds unlikely if you used 6 interfaces but did you use a different wire for all of them? you’ll have to do a process of elimination to be sure whats causing your problem. I upgraded to high sierra recently after putting it of for ages thinking it might speed up my stuff slightly and its been more problematic. not sure if you can downgrade mac OS but id imagine its doable, you could try that. you wouldnt be missing much from high sierra unless youre really into siri haha
are there high ended usb cables for this sorta thing? Or just regular usb 3 is good enough?
Midas mr18 (recording in DAW only) or the A&H Qu-SB (can record to USB drive) are two other options… what I dislike on those stageboxes that they didn‘t put knobs into the device…
Regular cables should be fine, I’m more thinking yours might be faulty