Sadly i did not make a “You finished the game!” button. If you bought the last track and finished the loop, you can consider the game finished and all content past the last track piece as “post end game”
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Thanks for the reply. Then I guess yours is the rare jam game that isn't super short but that I definitely exhausted all content for! (And then some — I had a million trophies and extremely high stats on every vehicle before I quit.)