It's really just the legendaries that are a problem. I now have every autoshow buyable car except for 7 of the most expensive legendaries. Of the wheelspin exclusive legendaries, I'm missing 5, I have every non-legendary wheelspin exclusive. The game will give you the wheelspin exclusive FEs and non-legendaries reasonably fast if you farm wheelspins, but the legendaries are tough because it doesn't appear to have any preference for giving you wheelspin exclusive legendaries, it seems to just be one big pool of legendaries you don't own that it can give you. And you only get a legendary roughly 1 in 100 wheelspins, or 1 in 33 super wheelspins. Those numbers are very approximate, I haven't tracked exact numbers, but I know if I farm 999 skill points and use them on Revueltos, I might get 15 spins from the XP, 75 spins from the masteries, 25 super spins, and you get some extras from spending the skill points, say 10 extra spins, so 100 spins and 25 super spins in total. That gets you 1 or 2 legendaries. So you want to buy the cheaper autoshow legendaries as soon as you're able to, to shift the pool as much as possible to the most expensive autoshow legendaries and the wheelspin exclusive legendaries. And if you can find any in the AH, that's helpful, obviously, I got a 288 GTO for 2m, and there were plenty of them, I've no idea why when there were no 599XX Evos, Venom GTs, Apollo IEs, Koenigsegg Ones, or Valhalla Concepts, the remaining 5 I need after getting the 288 GTO.