After feedback, I've changed how levels are unlocked.
Each requires a certain number of Golden Hubcaps (Medals). But reaching the finish line nets you five of them, and there are 10 more that you can get in each level. Thresholds are generally set so that you'll have to beat the previous levels(s) beforehand first, but the Hubcaps essentially let you skip ahead. They'll be good for skipping past troublesome levels, once there are more levels. There might be some rebalancing and UI tweaks in the near future. We'll see.
Expect more game soon. And bug fixes.
Happy tiring!