Route Advice Eastern Washington
 

I posted this first in the Tandem forum, but got essentially no response, so please help:

Stoker and I are thinking about organizing a trip through Washington wine country, maybe next year. Probably end up being a group of 6-10 riders. Thinking, just for instance, of maybe Winthrop to Walla Walla. Would like plenty of good hard riding (nothing over the top here, say 50-80 mi/day, less than 5000 ft elev. gain/day), but with opportunity to do some wine tasting. Usual effort to avoid crappy roads, busy roads, dirt roads. Maybe 10-15 days duration? Probably with van support to haul suitcases from one lodging to the next. I'm generally familiar with the area, but am hoping to get recommendations for specific routing: roads to use and roads to avoid, places to stay, must-do restaurants, etc. If anybody has done this or something broadly similar, I'd love to get advice, routes/maps, web links, whatever you care to offer. Even advice on smaller bits like "be sure to include _____ road to Grand Coulee Dam and ...".
Thanks in advance.