So... first regarding the sunroof. I've spoken with several tint professionals and they hear this common misconception all the time.
In short it comes down to the type of glass that is used in sunroofs and not the tint...
here's a snippet from a tinter's website
https://www.teamacme.com/tints/is-sunroof-tinting-safe-learn-all-the-risks-how-to-avoid-them/
Next up the factory "tint"...
The factory tint does not have the same properties as a film. These films have the added bonus of UV and IR transmission blocking and reducing the total thermal energy. The factory 'tint' is just coloring to make the window darker. So, there is still an added benefit to using a film on top of the factory tint. If you are worried about going to dark, go with a 70-90% film.
As always, your best resource is your local tint professional. I highly recommend Hot Spot Tinting in Marietta, GA.
https://hotspottinting.com/index.html
Or.... dive head first into the tintdude.com forums.