I like the isometric pixel art. Fighting against the boss with this battle system also felt very fun! I like the attack that destroys the platform, I thought it was clever. A suggestion I'd have for an update is for projectiles to have shadows shown on the floor, so that you can tell what lane they're in.
I hadn't read your GDD before playing and felt that Mikey's legs felt very Megaman-esque, but after reading it, I see it isn't unfounded. Mikey's design also reminds me a bit of character designs you'd see on very old cartoons, with the gun having the eyes and all.
Only thing I struggled with a bit was getting the arrows keys to work, but after I fullscreened the game and opened and closed the console, the controls started working. Other than that, the game works really well. This is good, especially for a first game submission. My first game submission was full of softlocks, so I speak from experience lol.
This is a very good first step and I hope you had fun putting it together. Keep making stuff!