Check the roof for roof vents, gable vent or soffit vents. Get in the attic and check for insulation. If you have soffit vents, make sure the insulation isn’t covering them. If you have roof vents, make sure roof is cut under them for air movement. The vent fans with temp control are great. I have a plug and play that is set up with blue tooth, and I have a hardwired that has a dial thermostat, both work great. There are also solar powered roof vents to help pull heat out of the attic.
As said before, insulation on the doors can help. They make kits to insulate doors, but they can be pricey. Check the walls for insulation as well. If there is no insulation in either the attic or walls, you can have someone come do blown in or spray insulation. Blown in is much cheaper. Some businesses you can buy the blown in and they will loan you the machine for free. More labor, less cost. Spray foam is expensive, but it can be great. Hire it out or buy the kits, the temperature has to be right for it to stick to the wood properly or it can separate.
A portable swamp cooler can go a long way to making it more comfortable for a relatively low price.