This got me a little further, but now I'm stuck at getting the error "Unable to copy symbols from this device".

This is one of the most amateur mistakes I've seen from Apple... and they've made a lot of amateurish mistakes!

We don't all want to upgrade. Many of us would stay on Mojave for awhile... a lot like how users stayed on Win95/98 for many years and in fact they are still used in some poorer countries. I saw several Win98 computers running in an orphanage library in Angola just 2 or 3 years ago. Mojave is my Win95. :D