At a minumum, put a couple of those
turbine vents up there, WITH (very important) some
soffit vents under the eaves. Air leaving the top doesn't help much if there's no air to come in and replace it.
It also promotes natural convection cooling - hot air rises and goes out, and cool air comes in at the bottom.
Next step would be a
ridgeline vent. Since it lays over the top of the other shingles, it should be a good retrofit option. You'll have to cut a slot along the ridge for air to come out, probably with a circular saw.
Then I would consider a solar-powered fan.
Any of these would be doable with the existing roof in place, though you might need a couple replacement shingles right at the work area.
How's the insulation on the attic floor? You felt the heat - it wants to come in through the ceiling.