re: Home Insulation/New Windows - Advice needed

Before you spend any money, check what you’ve got....
Go into your attic with all the lights on in your house. If you see daylight below hit it with some foam.
Are all of your doors sealing properly? Fix the weather stripping if not.
Check any place that has a pipe coming through sheet rock - sinks, toilets etc. Spray some foam or caulk it. Some foam isn’t safe to use on hot pipes (gives off fumes) so be careful there.
Check that your oven vent is working properly. They can get stuck open and the air gets pulled out.
If you have recessed lighting make sure it’s insulated with “top hats” and foam around the top hat.
Get some foam insulators for electrical socket and switches. Foam around the box too.
You can also spray foam around the top cap in the attic before adding insulation.
Rent an infrared camera and walk around your house identifying hot spots and address those where needed.
If you get a window salesman they will sell windows as a solution. Same goes for insulation or HVAC. Remember, “If a man has a hammer everything is a nail”
You very well may have a problem that requires new windows or additional insulation but it’s best to address air leakage as well as it’s cheap and in some cases free.

...if one room gets hot consider blackout curtains or reflective tint on the window (foil works too if you’re trashy and DGAF).