Sincere apologies for not replying sooner, life got in the way!
Thanks all for your super detailed responses, they're really helpful, and have thrown up more exciting questions!
The latest plan is to stick with the PPP route through Yellowstone, then take a rest day (potentially in West Yellowstone) and hire a car and drive down to see the Tetons. Granted it might not be as majestic from the window of a car as it would be from a bicycle, but we're keen to keep the total mileage as low as possible as we only have 90 days to finish the tour (visa reasons) and want to stay at or below a 50 mile/day average. Judging from Street View images it looks like we'd be on mostly narrow-shouldered roads for about 80 miles on the way into West Yellowstone. The 'ride early' tip sounds like a great idea (well maybe not the waking up at 6am part, but having emptier roads sounds fantastic). I like the idea of doing ~40 miles in to Canyon Village one morning, spending the rest of the day there, and then the next morning doing ~40 miles over to West Yellowstone.
jamawani, thanks for the tip on avoiding Columbia Gorge - I've long been keen to see Mount Rainier NP but had kind of assumed it would be too much climbing for this trip. Turns out it's actually not that much more than the Columbia Gorge route. We're not especially wedded to ending up in Astoria (just needs to be somewhere we can catch a flight back to the east coast) so if there are better routes to the coast from Mt. Rainier NP, I'm open to suggestions! I've mapped out two potential routes for this final section of the trip - one following your suggestions and going through Packwood and Cathlamet to Astoria (
https://www.strava.com/routes/2931209667759741260), and the other incorporating more of Mt. Rainier NP, heading through Eatonville and Rochester before ending up at the coast near Aberdeen (
https://www.strava.com/routes/2931203837469186250). Any insight on the best places to finish the trip in south WA/north OR? Also would you say the road to the south of Mt. Rainier (Paradise Rd I believe) offers significantly better views than the road to the east (WA 123)?
Also here's a link to our current full route map, for context:
https://www.strava.com/routes/2927621892056455372. Any additional feedback welcome!
Thanks again,
Steve
P.S. That photo of Bighorn looks mouth-watering