Here comes some more feedback since you asked for it :P
The game has a lot of charm and even more potential charm. The game is very cozy and you managed to create quite an atmosphere with the idea and the music, it honestly reminded me a bit of Firewatch! The vibe is strong and pleasant and the core idea of customizing fireworks is super unique!
The biggest issue rn are visibility, readability and player feedback. As a player it's not super clear what you're supposed to do and what the goal is. There's very little guidance, so adding some type of narrator / tutorial, or just gentle onboarding would help loads!
Volume sliders would be very useful. The music is a bit loud by default and it would be nice to start it at a lower volume and then let the player turn it up if the want to.
I like the overall progression structure, the way you assemble the fireworks and then the ending with the cute firework cutscene! That flow works well.
However, the UI has some usability problems:
-The handles are hard to notice since they are almost the same color as the background
-The feature of the handels looks visibly pleasing, but in practise it feels a bit unnecessary to drag the small boxes back and forth.
-Right now the separated boxes sometimes overlap when you try to customize.
-I would prefer if the colors were visible upfront. Maybe customization could be split into two clear steps: Choose color -> Choose effects.
The little effect and animation that appear when you press a color is nice, but adding some type of indicator on what you have selected would be good for player feedback. For example: highlighting the currently active color / effect on the UI as well.
At the moment there is also no replayablity. You "could" play it again but there is no strong reason to. Adding a goal-driven system would help a lot. Maybe some type of small mission where someone requests a specific type of firework, and you have to recreate it based on clues or requirements?
To bring everything together, a light narrative or story could really work well with this game. For example, some poetic story. It could maybe be a bouta parent and a child on New Year's Eve, talking about life while creating fireworks together.
The player could figure out favorite colors or styles through dialogue and try to create the "perfect" firework. Something like that could give the game more direction and emotional impact. With a story you would get invested in the making of the firework and also the last scene would have even more impact!
I know some of this is way over scope, but I'm sharing it since you asked for more feedback :) Overall, this is a super unique idea and with some more polishing, personality and direction, this could be something special!