I'd like to offer you an alternative to both Plans 'A' and 'B' as well as "stick[ing] to one of two main routes". There are always alternatives, especially since you have six full days to make the journey from San Francisco to Kansas City and don't want to do it "just on a 4 lane highway all day". So pull out your maps and follow along...
From San Francisco take I-5 south through the Central Valley to CA-58 east through Bakersfield to Barstow and I-15 north through Las Vegas to St. George UT. This will be the bulk of your four-lane autoroute driving; it will let you get a good start on your drive; and since this is a flat, mostly colorless, and basically desert portion of your trip, just put down some miles at the start. I will just note that you will be passing north of Edwards Air Force Base where some of the US Air Force's most modern planes get tested.
Next up are some of the southern Utah national parks. From just north of St. George take UT-9 east to Springdale and a stop at Zion National Park and then continue to Mount Carmel Junction and US-89 north. You can take a short side trip via UT-12 east to Bryce Canyon National Park before returning to US-89 north to I-70 east over the San Rafael Swell to Grand Junction and the Colorado National Monument.
Next up, take US-50 through the Rockies and the Black Canyon of the Gunnison and another possible side trip to Great Sand Dunes National Park on your way to Colorado Springs. From here to Kansas City, you'll be on the High Plains and again scenery will be at a premium. I-70 and/or US-40 offer the straightest shot, but if you want off the autoroutes for this portion, consider US-36 between Denver and St Joseph MO or the more direct CO-94/US-284/US-40/US-83(north)/US-24 between Colorado Springs and Kansas City. Typically, these will be two lane roads with fairly high speed limits through ranching and farming country with the occasional drive through small towns to give you a feel for the countryside that you'd miss on the Interstates.
All of the above could be driven fairly comfortably in four days of solid driving, leaving you a couple of days to spend exploring the parks and scenic areas along the way.
AZBuck