As for the bullet sound... it might be the volume, or me just not liking the sample. The thing with these sound effects is that when they're great is precisely when you don't notice them, and they still feel nice. The spread is alright, to be honest I think it does fit the game, but it's part of the elements that make the bullets not feel "strong" or "powerful", which is ok if that's not the feeling you're going for. With more powerful bullets one might "choose to make a last stand", instead of "running away and shooting behind you".
In terms of games like these, and some related genres, each enemy type can serve a specific purpose; an enemy that always follows the player is one that forces the player to move away from them, especially if they are hard to get rid of like in your game (since big ones create small ones when they're defeated). A very simple enemy type to add to compliment this is the stationary enemy that literally does nothing, or an enemy that just walks randomly (in circles, in a line, etc). In any other situation they would be completely useless, but in a game where the player is constantly moving away from a horde of enemies, enemies that don't move towards them tend to get more in the way of players, and they have to focus on dodging them while running away. There's other enemy types that you can use for various reasons, such as area denial, making the player stay still, making the player dodge side to side, etc. Eg: what purposes could an enemy that shoots bullets at the player serve?
One of the other things allowing the "main strategy" is the arena itself having borders. Walking in large circles very close to the border is very safe since enemies will only spawn within the arena and will naturally join the horde you're herding. You could either remove the borders, make it so that the player doesn't want to get anywhere near them, or incentivise the player to go near the center.
In any case... This was a very solid game for a game jam, it's really impressive that this is your first time making a game in such a short amount of time. Jams require really good time management, learning to prioritize things, and so on, and you've proven you're good at all of these.