Mickey @ TST is not something I would FP+ for one of my first 3. If you are sure you will be in the park after 6pm his line shortens considerably. You can also be assured that once you use your first 3 FP+ you will be able to book a FP+ for Mickey, he should always have availability. Especially since your trip is during one of the lowest crowd levels of the year you shouldn't have a problem seeing 15-20 mins or less posted for him in the evening.