Great game! I got 60 deaths and just under 10 minutes haha!
Aside from what you already know (aka adding sfx) I will say the boss at the end is a bit difficult and the music bugs and plays both the level music and boss music at the same time. Individually they are great, together... not as great. There were also some collision issues towards the end, but I'm not really sure how to describe them all too well.
There was also a point right after checkpoint 3 I believe where I would transition over to the next screen and immediately die because there was no ground there to catch my fall!
But overall I loved this game, the aesthetic is great and massive kudos to whoever drew that amazing thumbnail, y'all did great!