RTD Park-and-Rides have a loose no overnight parking policy. I say loose as I've known people who work in Downtown Denver and have parked their car at a Park-and-Ride, went for drinks after work and couldn't drive home. They didn't get towed but they did get the car the next day. For your purposes, I think your car would get impounded if you used one.

I'm not sure if a parking garage in downtown or in the DTC would have an average daily cost of $5/day which is what Pikes Peak charges. I agree with your consternation about Pikes as I've waited over an hour at DIA waiting for the bus and no longer use Pikes unless I'm departing and arriving during a weekday. The bus' seem to run more consistently during the work week than weekends.