Congrats on completing your first jam game! I actually played it all the way through and enjoyed it, more than I have some much more polished games. You had creative ideas for each boss and they increased in difficulty which was really cool to see. I saw someone already mentioned the Juice video and that was my first thought as well -adding feedback - sound effects, flashes when an enemy is hit, knockback etc. will go a long way to improve what's already there. There's lots of great resources for free music and sound that'll help you out. You have a great start, keep going!