Hi, I have a 21.02 port but the dts file was pretty bad on it (plus some other files), over the last couple of days I've improved it thanks to help from @bobthebuilder.
I've just pushed my latest dts file which has the firmware partition, which should fix sysupgrades (not sure, the verbose option didn't show any info that would help in finding the cause but that may have been it).
I think you still need to do the mtd method if going from 19.07 to mine because 19.07 lacks firmware partition.
From what I read up here our only option would be to find the RSA key, soldering or the leaked fw method. I'm not sure if disabling the RSA check in the bootloader is even possible.