Thanks for the suggestion!
The gameplay already only requires one button, but it's definitely possible to make the entire game fully one-button especially now I've recently managed to free up 10 events in the free version of Construct 3 by reworking things so you can potentially cycle through and select unlimited number of balloon colours and cue sounds without using up more events.
I've actually made a few one-button games before, such as ONE TAP CONNECT, which can perform a few actions with a single button. I think I'll make this improvement very soon.
My current idea is that after your score is announced, you could wait for the game to restart or hold a button until the countdown ends to quit, and make it so you can skip somehow eg if you know you did terrible.