Hey thanks for the response and proper feedback! I haven't been looking at this project in awhile but you definitely pointed out some stuff that needs fixing. I think I will come back to this project and rectify that stuff along with adding a small bit of new content.
Sound effects missing is definitely something the game needs. Even if I don't make any other major updates, I'll definitely put that in.
The biggest issue I noted in working on is the collision stuff on props as well as when a bunch of zombies get close to the player at once and sometimes pushes them through wall geometry. Hopefully I'll find a decent fix for this.
I do have a system that makes the zombie spawns favor spots closer to the player, but I think you're right, I need to increase the strength of it to prevent the boring early game, especially with the slower zombies.
As far as additional content, I'm considering making a procedural map system or just hand designing a new one. This entire project is very much a design/programming practice project for me and I think designing procedural levels would be a good exercise.
I'm glad you like the weapon upgrades and unlocks. I'm hoping to insert some moderate build-crafting with the weapons, and that upgrade system is the primary method for that.
Thank you again for the feedback, I greatly appreciate it and hope you'll give the game another look after an update.