OK, I worked it out. Turns out that Oracle Linux 8 doesn't like the syntax used in the published MTD backup script, but Ubuntu was quite happy to run it.
Also, in order to login via SSH from Ubuntu, I needed to use BOTH algorithm options chill1Penguin mentioned in step 3 together in 1 line like this:
ssh -o hostkeyalgorithms=ssh-rsa -o KexAlgorithms=+diffie-hellman-group1-sha1 root@192.168.0.1
Using these 2 things, I was able to complete the full backup of the original stock firmware before I mess with re-flashing with OpenWRT. I have posted another thread to explain this, just in case anyone else had the same problems.