Boy, is this a long topic or what! I started reading through it, but I gave up when I realized I’m only tenth of the way down.
Anyhow, in case the situation has not resolved itself yet, I had a very similar issue with a registry that has been in use since Julia 1.0, and continuously upgraded. I posted a similar question here, although I did not complain primarily about the time it takes to udate the registry, rather than the amount of network traffic. Still, the apparent solution to both issues was to simply delete and readd the General registry. I stopped having issues with the updates since then. If your issue still persists, it’s worth a shot IMHO.