Slightly expensive experiment, but I guess I'll see which of these work, and return any bad ones (incl. https://www.amazon.com/dp/B073R6XJND which at 9$ for 3 was simply too good of a deal to be true...).
In other news I think I've figured out a better bootcmd (it actually honours partition table layout) - though it needs a bit more testing. Keeping fingers crossed that I don't end up with a brick when I start mucking with partitions tomorrow.