This is not necessarily true. If one is able to snag a FP in the first few minutes after park open, the return time will be within an hour. At that point, you can go grab a Cars FP before using the GotG FP.
Yesterday, a Thursday, we were in the turnstile line by 7:25. At 7:30, they allowed people in the park to get in the FP line. We were within the first 150 people to get FPs, and pulled our FP at 8:03 with a return time of 8:55. At 8:55, we were able to pull another FP.
My suggestion to the OP is to nudge your arrival at parking up as close to 7am as you can, so you can be as close as possible to the front of the line at 7:30 when they will likely let guests into the park.