So I'd have to say OT since he has quite a long day and doesn't get 2 long naps in and most of his A times are on the long side even after a short nap in the afternoon.
Would you consider doing an earlier BT around 6 or even 5.30pm?
Have you tried resettling him when he wakes from the first nap on the school run rather than keeping him up another couple of hours? He is probably OT from that wake up onwards. Or any chance of keeping him awake on the school run and putting him down in the cot when you get home? If you could get him on 2 long naps you could do away with the third nap and make his day shorter thus giving him less total A time. Also have a look at the 3-2 nap transition sticky which has more info on dropping the third nap.
All about the 3-2 transition- 5/6 monthsYou could definitely try weaning the dummy if you think he has become dependent on that. Does he have a blankie/lovey he could hold and suck if he wanted to? How do you feel about weaning the swaddle? I wouldn't worry about the white noise as you can just leave that playing for the whole nap. Do you? White noise really helped my Ds2 and he didn't even notice when I weaned it cold turkey when he moved into his brother's room at 15mo.
The other option is wake to sleep about 3am.
What do you think?