For the record, changing branches and then doing updates got me closer, but still not all the way there.
@stephenw10 said in SG-4860 updates are broken after factory reset:
Just open a ticket. You don't need active support to get the support image: https://www.netgate.com/tac-support-request
Thanks for pointing this out. To contribute to this for others to reference, in the FAQ's (https://www.netgate.com/support/frequently-asked-questions) there's a "What is TAC Lite?" section, which includes "Customers who purchase a Netgate appliance do not have to purchase a subscription and have TAC Lite free of charge for the life of the appliance."
I submitted a request, selecting "TAC Lite" for the Support Level, and they did indeed respond with temporary links to download an install image.