Thank you Jay – your troubleshooting steps above all checked out, so while I was in there, I just cleared things out and started over. This time Discourse downloaded the MaxMind dbs, and I can now geo-resolve IP addresses.
All I can guess is an oddball glitch with my MaxMind account, or maybe a copy/paste error with the original license key.
Your efforts ruled out other issues, so I’ll mark that as the solution. 