I appreciate your efforts for making this game. In fact I was unable to brainstorm for ideas on "bottleneck" and I feel bad for not trying it. You did a great job I mean. I wanted to give some suggestions on this build. First of all the audio while changing scene start over again. For that I suggest you to use what it is call as 'Singleton pattern'. Using this you can make the audio clip without starting over while changing scenes. So try to implement this. Other this is the main menu alignment. Make the buttons align in center and anchor the button(s) on whole window. Last thing is the scoring system. Was it supposed to show time or score acquired? If it has to be time then keep it for 1 sec. If you wish. No big deal 😅. 
Overall it was a great implementation for theme. But as per optional theme challenge I guess not. (don't take it seriously😜)
Great work 🤘 Good Job 👍