I agree with your take on the afternoon
most walkups (clearly not all ... but the ones I've explored) have a quiet period between the lunchtime rush and the happy-hour rush
But they're still busy at 1pm with the late lunch shift, which tends to begin tailing off around 2pm.
Best time is 2:30 to around 4pm
BTW she won't usually get to eat until around 3pm when she gets a take-out delivery ... so take the girl a snack or a bag of fruit, and you'll get a very appreciative welcome