Excellent work!
I think the core gameplay has great feel, found it oddly relaxing to go around killing zombies, vibing to the music.
Controls are responsive and the aim assist with headshot option works nicely.
It has somewhat of a beat'em'up vibe, without the button mashing.
A few thoughts:
I did not find the jump very useful, when I jumped on stuff, it made hitting the zombies more difficult (melee), while they still could gnaw at my legs.
I did not undesrtand what advantages crouching brings as well.
The camera worked well most of time, but got in the way in a few edge cases (some thin walls did not go transparent and the character was obscured).
I did not use the aiming with RMB, prefered just mousing over freely and shooting.
Brainstorm (trying to avoid being too scope creepy):
Replace the jump with (or just add) a dash or roll maneuver.
Add a heavy melee weapon that can hit and tumble a bunch of zombies
Add low obstacles, you can pass when crouching, but zombies stay stuck
Overall, you got a very nice core, goood luck with the next steps!