Your attic temperature should be at or near the outside air temperature if it is well ventilated and does not leak warm air into the attic from below.

At the moment the attic floor is properly insulated, and has a 3/4 plywood floor on top of that. I am going to install an attic tent for the pull down attic stairs. What else can I do to prevent heat leakage into the attic?

Put an air conditioner in the attic. Add weatherproof slit sheeting of some sort to the top of the roof 6 inches to 1 foot above the roof that or solar panels. A solar attic fan that way if the power goes out you will still have it operating. Although most of what I said would be best for desert communities. Yum dessert!