I have the same problem with a different setup. Half finished walls. Nothing up except the roof. No insulation on walls or ceilings. 15 degrees hotter than outside.
Lack of insulation on roof and vents that are kind of blocked by the finished walls is my issue.
Solutions I've thought about are putting a vent up high, by the ^ of the roof. Putting a fan near that. Stapling insulation to the joists, just under roof, to keep some of that heat out.

Out of left field solution: get a heat pump water heater. It will be a lil loud tho.