I really liked the idea of the game and how the enemies would drop a part of themselves for you to use as ammo! The upgrade system is also fantastic and you did a great job and providing a variety of upgrades! You said you wanted feedback so here are a few things that I would suggest:
1. I feel like ammo is a little too scarce. Most of the time, the ammo you pick up is shot out in less than a second and you're immediately looking for more ammo. You are quickly overwhelmed with enemies and there isn't much you can do because there is such limited ammo on the map. I see a few solutions, you could make the bullets do more damage, make enemies ALWAYS drop ammo, or you could make it so each ammo pickup gives you more ammo.
2. I feel like having to click the ammo takes the focus off the main point of the game, which is trying to survive and run away from the enemies. I think making it so that ammo is automatically picked up when in range would be a good improvement to the game feel. Also, for players using a trackpad instead of a mouse, it can be difficult to quickly move the mouse and click on the ammo.
3. I think the player is a bit too big. First of all, the player feels a bit out of place in the world because he is so big. Second, if you are overwhelmed with enemies it's really hard to try to weave your way out of the crowd, so you kind of just have to give up and accept your fate.
These are just my thoughts nd you don't have to change anything if you like the way it is. I think this game has really good potential! Keep up the good work!