Sloped ceiling - remove hot air with exhaust fan
I usually advocate for a small mini split air conditioner in your case. For less money you could go with a portable unit in the master bed. The problem with adding a new duct is it will imbalance the system if the rest of the house is good. Also when you go to heat in the winter you will get twice the heat up there. As far as a fan up high, I think you will need a whole lot more air flow than a bath fan can move. Also you will then be dumping all your cooled air outside. Shouldn’t work well but I have seen things like this that shouldn’t be a good idea on paper work out due to other factors.