For stimuli, several games had different approaches. For things like clicker games, it had a distinct popping sound and the change in the scenery as more and more elements generating currency was added. For other games, they used vibrations per click.
So it truly depends on how you want your games to feel. Some games took it a bit too far with the stimuli by adding popping sounds that were effective at first but when you hear over 10000000 popping balloons, those sounds will follow you forever.
A game like this feels like it builds on people's love to see big numbers grow. It's its greatest strength. I wish I could give a more specific input but I'm not much of an expert in this type of game. Maybe try a few around online and see what makes them stand out and try and apply them to your game and its unique vibe. The game you have is very calming and relaxing. I feel it will do so much good in the future if given the chance and opportunity to grow. Its beautiful, hand drawn adds so much to the charm. Again, please keep up the amazing work!