@mdzprocter I'm not sure who sent you on the fool's errand of probing /dev/mtd0 (and running strings /dev/mtd0 | grep -i error on it), since that contains your bootloader. Thats a read-only partition.
As @anon89577378 pointed out your device uses DSA as of 21.02 so you should at least wipe /etc/config/network prior to upgradiing from 19.07.
BTW, an online search on your issue points me right to this topic: