Fun Fact: Rhythm Heaven was not actually in my original concept.
My original concept came to me in my dream, where I was controlling a large panel with various buttons and sliders.
The idea of making it Rhythm-Heaven-like came when I was thinking for a good way to give the player instructions. Visual cues would be very difficult to follow when there are a bunch of buttons to press. I also didn't want to directly put indicators on each button since I want to make the player feel like they are actually doing some kind of job. Therefore, it was clear that the best way to give the player instructions is through audio cues, Rhythm Heaven style.
(And yes there were actually different button types and sliders in my original concept and you actually need to use them on the correct beat, but since it was my first time making a rhythm game, I decided to go easy and make every possible interaction a combination of single-press buttons.)