thanks for being gracious with the feedback! i always feel a little awkward sending unsolicited criticism, even if constructive, but i figured it could be helpful! if i write a lot about a game, it means i liked it and want to help the authors go further in the future. the changes you have planned sound like good ideas!
i totally understand about wanting a minimal UI. i didn't understand at all about the numbers on the cars. billboards would definitely be helpful feedback, though if you wanted to get fancier, one idea might be that, when a car dies, you make its number fly up from the bottom of the screen, then decrement it while the player can see, before it fades out. it'd probably help too if each number was a different color, so it's extremely obvious that the number starts as the number of the car you just killed, then decrements to the number of the car you're about to see.
now that i know about the car numbers i was able to make it to the second level! i did bounce off that level eventually for the other two reasons (long straight segments and trying to make it through those traffic arrows with the braking cooldown is hellish). i did have fun though, thanks for making it!