I then invested in paid shed plans, and the difference was huge. I ended up building a storage shed with no mistakes and I felt more confident as I was using good quality storage shed plans.

Your saying , it remains at 81/82 degree without the use of your ac at a outside temp. of 94 degree ? And it stays at 81 degree with the ac unit going with an outside temp of 94 degree ? Invest in a large circulation fan, your analogy of temps does not make sense. It sounds like your garage door is the main culprit. Take a thermometer and stand it about 6 inches away from the door wall and see what the temp is , than take a reading at the middle of the room.If they are the same ,get yourself an additional ac unit or build a conventionall wall in place of the garage door. !

Not sure where you are, but where I am is like crazy hot too. My house is a 2 story and the garage door was not insulated. Exterior walls to the house are insulated by code. The wall in the garage that seperates the garage and kitchen is not. So, basically, the heat from the garage door fills the garage, then it transfers to the kitchen and also to the two bedrooms directly above the garage. My garage would cook all summer long and the bedrooms would cook as well. To help the house and garage stay cool, I bought 4 sheets of 1.5″ insulation from Home Depot and cut pieces to fully insulate the garage door. I have no gaps in the panels of the door. The only gap I have is at the concrete joint in the floor. I also bought an insulating strip that fills the gap between the house and garage door at the top and two sides. I walk into my garage and even thought it is 100+ degrees outside, I have actually caught myself saying how cool it is in my garage. It has also helped tremendously with the temp of my bedrooms upstairs. Insulate that gaarage door.