It seems like my issues were with the loader I was supplying - I found the one I used before from a backup but even that didn't work.
However, everything just worked when I let edl figure out the loader by itself without me having to specify one. I was able to successfully send the boot.img to my device, patch it, and then send it back.
However, I seem to have run into the issue mentioned by Renate here:
https://www.mobileread.com/forums/sh...6&postcount=21
It seems Onyx has kneecapped fastboot and I am not able to use it to make the Boox boot with the patched image. Has anyone figured out a workaround, or are these devices unrootable for now?
Thanks again!