Yeah, you're right that there's some empty space in the middle and end of days after you've achieved the combo. I considered unlocking the next sign immediately, but I think forcing players to slow down and just play/experiment with the flowers that they have available now (including the new one just unlocked during that day!) fits the chill, lazy vibe more. Or have a bit of time to just sit and watch the scene change, without even worrying about the flowers.
As for rhythms, I explored allowing "off-beat" clicks (like, eighth notes) as well, during a combo. It felt a bit too hectic for the pace I wanted though, and representing this on the combo meter was going to be a bit of extra work that I decided wasn't worth it. I also considered black flowers playing two eighth notes instead of a single quarter note as a fun twist for the final flower. Deciding what notes to play would have been kind of arbitrary though and the current five intervals of [root, third, fifth, seventh, octave] for the five colors (the same intervals used in the background chords) was hard to disturb. Maybe for a sixth color! All that said, I think there's definitely space for some more interesting rhythm here.
Thanks for playing and for the fun discussion both here and on your own game :)