Spray Foam Attic Above Garage

Sounds just some blown in loose-fill over garage would what you want at a much cheaper price point....Assuming you don't have a bunch of shite stored up there.

You could do it without pulling it all up. Or pay somebody to pull up the necessary flooring and take a trip with the money not spent.

I think open cell is used in that situation so when your roof leaks, it can leak through the foam too.

I used a garage door insulation kit that helped (my garage faced West), although I didn't measure how much. It was cheap enough that even dropping a degree or two made it a non issue.

For the attic directly underneath the roof decking, isn't that what radiant barrier is for? It's been the default in every house I've bought in TX built after about 2009 or so.