Just posted this earlier yesterday. Basically I've automated what
@warp9engage said, but I'm using the Mercedes CLK-GTR instead because it has the x7 multiplier perk and Speed Skills Boost. This means you only need to rack up about 71000-72000 skill points rather than 100,000. This one is focused on acquiring Speed, Air, Kangaroo, Clean Racing skills.
A faster way based on my experience in Horizon 4 is to look for a Drift Skills Boost car that has a Skills multiplier, Air and wreckage skills. The reason for this is because you can clock in all increments of drift skills, and smash through small debris at the same time. This racks up skill points at such high frequency even the ticker can't keep up with you. If you find a place you can slide left and right (with drifts that last long enough to register "Ultimate Drift", rinse and repeat in the other direction), while clobbering through obstacles without losing speed, you can reach your target goal of 500,000 skill points in a minute or two. The only reason I didn't recommend this at first is because this occupies your hands.