There are 4 types of PPF, matt, gloss, self-healing matt and self-healing gloss.
I had self-healing gloss on my last car, I wished I had ordered matt as it's much more unusual and gives your car a unique look. The car did still look like new after 2 years.
I concur with your theory on dark film, fitting a film on the underside may have little effect as any heat reflected is likely to be absorbed by the roof itself. I think the fix is to fit film on the outside that reflects the heat of the sun and stops the glass panel getting hot when the car is stationary, there are specialist films available for this.
I now have rolls of photochromic window film for the windows, I want to try it by holding it first as it needs UV rays to work, I have read that a lot of car windows remove UV so if it doesn't work I will go with a good ceramic tint after a while, I never block more than 40% of the light as I find it awkward at night.