I... I was... I would have.... FUUUUUUUU--
Ordinarily, I would absolutely have tried another USB, and I actually was trying a bit ago, but that drive was being funky and wouldn't connect to Windows well and I gave up. I completely overlooked trying Balena Etcher (Rufus has always been rock solid for me so I never bother thinking aoub it) but NOW THAT YOU'VE SAID IT I feel like I remember needing to use it for CORE back in the day TOO! I am an IDIOT!
Anyway, I grabbed a third USB, Etcher, and a new copy of the .iso, and set it all up again, and wouldn't you know it, the damn thing worked. Installed HexOS to the SSD, plugged in the HDDs and went to boot, and the box picked up the data drives to boot to. I thought I wasn't out of the weeds yet, but on a whim I tried re-enabling the B120i and going in and deleting the RAID array on the drives but keeping the single "array" on the SSD, which allows me to set the logical array as bootable in the B120i. Rebooted and...
LOOK! HexOS!!
I am as much at fault as any part of the technology in this thread, but in the end, through all that crap, and with your help, we got there, so... THANK YOU very much!!