Meguiar's makes pretty good stuff from my experience with it. I used Star Brite from auto parts store, which is same stuff as Scamp
sells. Not impressed with the protective shine life span of that product. Using the rest of the bottle on inside FG where it does not have to stand up to weather.
I would not use anything with a "protective" aspect if I wanted to finish with Red Max or Zep. Any "protective" layer left behind during cleaning/polishing would have to be fully removed before application of those finishing waxes.
With "yellowing" I suspect old wax coat. Easy enough to check and real easy to wash old wax off.
Rub down with oxidation remover or Bar Keepers Friend might also be a good place to start. Do a small area with different approaches and see what you see.
When you like what you see.... do that.