Hello and welcome to RTA !

Yep, with 6 and a bit days each way it should be quite relaxed, although you won't have much time to hang around in anyone place to long, other than your 3 day's in Chicago. Heading out the way you have mapped and coming back along a more Southerly route will let you see more of the country.

One thing you need to know is that route 66 was decommissioned years ago and replaced by Interstate. There are original sections you can pick up, but in places it can be tricky and time consuming.

Chat with your mates about budget, at what pace you want to travel, what to see etc and then decide what travel arrangements work best for you.

Have a good look around the RTA site, you will find tons of info and road trip ideas.