Get/borrow a thermal camera, and use that to see where the heat is coming in. Some libraries have thermal cameras, but that might be rare. You can also get very cheap and nasty thermal cameras from Aliexpress for $60 AUD

You need some kind of vent fan to blow that air out of there. Like a thermostat attic fan for example, In the garage. I’ve seen people crack a window if you got it. Crack the garage door or back/side door when you’re home. Nothing is going to get rid of that heat except some ventilation of some kind.

You might’ve insulated your garage door, but did you weatherize it? You can get decent weather stripping to go along all 4 edges of the door to block off the cracks. As far as insulating your garage ceiling, it’s something I’ve considered too, but one thing I don’t have an answer too is how that would affect the airflow or code compliance for my gas furnace and gas water heater, which are both in the garage.

Also, when we did an exterior remodel, we discovered there was no insulation in the exterior garage wall, only in the walls shared with the interior of the house, so you may be contending with that. We had that wall insulated when we did the renovation, and it made a noticeable difference. I’ve also planted trees to shade that garage wall, otherwise it gets blasted with afternoon sun.

If I had the energy and time, I would put batt insulation above the garage ceiling and also replace my attic stairs, which are old and poorly sealed. A mini split AC would also be pretty sweet.

You might be interested in watching videos by Matt Risinger on YouTube, he does a ton on insulation projects in 70s-80s builds in Texas

Put in a radiant barrier. A radiant barrier is a layer of metallic foil that reflects up to 96% of radiant heat, assisting in the energy performance of a building.

I have an attic that gets direct sun until the afternoon. It gets quite hot. I had easy access to the rafters so I used radiant barrier, and the difference is very big. As you’re putting it up you can tell that it’s blocking the heat standing in an a covered vs uncovered area. In subsequent days when it was all up it was obviously cooler. It’s still hot but not unbearable.

Radiant barrier is more expensive and fiberglass probably would have worked just as well in this situation, but I didn’t know enough about air flow in that space to tell whether fiberglass would impede anything,so I used radiant barrier and left a gap at the bottoms and tops. It is very easy to install. Fiberglass wouldn’t be too hard either, but the barrier is daed simple and there’s less volume to move around.

In general, my experience say it’s going to help, and whether you do fiberglass or radiant barrier is up to you.

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.

If you can cool the garage, insulating the ceiling will help. Even if you run a fan to pull in cool air at night and then try to hold it in, it’ll help.

I did a radiant barrier on my rafters to guide heat up from the soffits to the ridge vent and then insulated the joists. Blown in insulation is pretty cheap. While doing other work I also insulated my southern facing garage wall and of course insulated the garage door (and sealed the edges). This made a noticeable difference in the temperature of the garage. I can turn a couple of fans on in the garage and leave the door to the house open and achieve a very reasonable temperature in the garage if I plan on being in there for a length of time. This helps in both summer and winter.