While this was a fairly easy achieve to get when the game was in early access, there's now a LOT more packs to unlock. So, you won't be getting this achievement quickly.
All the skins add up to around 420,000 coins (not including the price of the cars) and you get around 200/300 or more coins per song (including either getting a discovery bonus or beating your opponent).
So, the best ways I can think to speed this up is
1. Generate tracks for every obscure song you can think of. You get 100 coins just for making a track nobody has played before, and you don't actually have to play the track to earn them. It's a much faster way to earn coins (200/300 coins per minute?) though you wouldn't get to actually play the game much.
2. Always play against the recommended opponent on the leaderboard rather than the global leader. You get 100 coins for beating your opponent, which means you could potentially play a popular song multiple times and you'll keep getting 100 coin bonuses until you've beaten the top score.