My thoughts are:
2 seats together so no one else to disturb. Being August there is a good change the flight will be very busy, so hoping to get a spare seat in say of block of 3 is reduced (you could book an aisle and a window and leave a gap – bit someone might end up in the middle).
You are probably going to be at the back of the plane so one of the last to disembark. Given the feedback on that queuing at Miami for immigration seems to for the most part very slow at the moment I’d be tempted to get as far forward as possible as you may save quite a lot of time being the first economy passenger off than nearly the last. I guess that depends if you don’t mind waiting or if you have to get off quickly.