You're on the right track
I am not an HVAC contractor, but I am a Mechanical Engineer. I think all of what you said could work; my recommendation is a temperature controlled gable vent fan. Should only open up and run when the attic is above a set temp. This is what I am planning for my house.
By the way are the areas that don't vent small less than 4 feet or large more than 4 feet? Are you sure insulation isn't blocking the vent?
Also to reduce heat gain in the summer you can buy metal paint (yes metal paint) and spray it on the roof sheeting underside. This reduces heat gain from radiation. People with metal roofing will have cooler attics then people with asphalt shingles.
Heat transfer from your home can occur by conduction, convection and radiation. It is typically modeled in terms of conduction, although infiltration through walls and around windows can contribute a significant additional loss if they are not well sealed. Radiation loss can be minimized by using foil-backed insulation as a radiation barrier.
Hope this helped.