Flat roof insulation options - worth it?

I am buying an older house in Florida. It is currently 94 degrees every day and feels worse. Sellers are replacing a flat roof over part of the house. There is currently no insulation there, just a bitumen roof. The replacement will use GAF Liberty product base and cap sheets. We asked for options to add protection (that we would pay for). This roofer says they cannot raise the decking and add insulation because they are not a licensed GC.

As an option, they say they can add a tapered system to give the roof some pitch with PolyIso boards and then cover it with a TPO system. They are telling us it won't add much protection because it is a smaller roof area (420 s.f.), but to me it seems that anything would help.

I welcome any thoughts- we do not yet have information on what the cost increase would be. Thank you!