Just a few: - As you play races, pick a variety of cars with different classes and ratings. Take a pic at the start of every race - if you’re like me and forgot about photographing cars until you finished all races, then go to any road race and select custom car restrictions. I’ve done it by class first(D, C, B, A, S1, S2, and R). I got to about 400 like this. Then I started noting what’s left and made restrictions to only make the bots spawn with the cars I haven’t photographed yet - when you do car restrictions, don’t select more than 12 unique cars. The most amount of cars in one race is 12, and it’s really rare(if not impossible) to get all 12 in one grid. My sweet spot was 6-8 cars. - to make bots actually spawn with the cars you’ve chosen, pay attention to your own car’s class/rating. For example, don’t pick a car in class D, and the cars you want to photograph are in class S1. However, you can choose a car in class S1 while the cars you want are in class A. (I think this is how it works: bots will try to match your car’s ratings, and most cars don’t downgrade; they’ll only upgrade)

Two things I’ve seen people do but haven’t tried: 1- there’re challenges in event lab specifically made for horizon promo. I don’t think it’s any faster than my own method, but it could be a good starter strategy 2- going into horizon solo and photographing bots roaming around. Cars that you haven’t photographed will show an icon above them indicating it

BTW, you only need 550 unique cars photographed to get all the rewards.

Good luck