There is insufficient wind bias to justify using it to pick a direction. I've been across the U.S. in both directions, and, no matter which way you go, the wind is sometimes in your face, sometimes at your back, and sometimes at your side.
You can start whenever you want, but I like lots of daylight, so, all other things being equal, I prefer to center my trip around the summer solstice. Also, some people hate the heat more than the cold, and others hate the cold more than the heat, so your bias might influence your start. I'm not sure how far north you're starting, but I'd probably start heading south on the Atlantic Coast in the mid to late spring as soon as the weather gets warm enough.
Some people don't mind riding on the interstate, but I prefer to avoid it like the plague. I don't like either of your two routes through Washington. I'd jump from the TA at Missoula, go up to Glacier NP and then stay on the Northern Tier to the coast.
In my opinion, 75-80 days is tight for the trip your plan, especially for someone who says time is no issue. Take a bit more time and enjoy yourself.