I try to chose sleepy roads whenever possible, and I do find it possible 90% of the time. Following an ACA route is really helpful in this regard, as well as consulting the traffic volume information in state DOT maps.
Sometimes, however, it is inevitable that you'll find yourself on a busier road. I do find, however, that 999 out of a thousand drivers are very courteous and accommodating. I try to get over the other one as soon as possible.
I agree with you about the funneling effect in the west, especially in the mountains. There seem to be a lot more roads in the east.
I think everybody has different tolerances for traffic. I hadn't really ever thought, however, about a "burnout" factor. Most people I know either tolerate traffic or they don't. I see that you got stuck on I-90 for a while in your tour. I avoid interstates like the plague, although I know others that like interstates. Perhaps because your trikes and trailers are wider than a standard bicycle, you feel closer to traffic. I'm sure you'll feel eager to get out there again soon.