I tried the first option above. At my friend's place, the USG seems to work just fine when I plug it in to his KPN connection - exact same settings, no packet loss.
This means that the issue exists only (1) on the USG, (2) with OpenWRT, and (3) at my house - e.g., with a different device, or with stock firmware, or somewhere else, the issue is not reproducible. I continue to be mystified.
I'll try the second experiment, maybe some time this week.