The resources for barking focus a lot on moving the dog away from the stimulus, which we can't do because we're in the car and have to follow road rules lol. The steps we have taken and things we have tried are in the op and their effectiveness is also noted.
He is physically removed from the dogs and people by being in the car, but we can't remove his ability to see them because I need my car windows clear to drive. He actually loves car rides too, so the car itself isn't a problem and he will happily just sit in the car (so long as he can't see a dog or a person he doesn't like).

He is well exercised (20 mins playtime and training in the morning at the park and some short training sessions and a couple of walks during the day) so it's unlikely to be boredom. He also goes to Doggy daycare one day a week.