It's very interesting to play your game. Power-ups are fun, UI update is good, now it looks much better and main menu is nice. I think that the game needs just more content and polishing in some things.
For example, sometimes I notice that bullets hit walls in a way, which doesn't feel right. Especially often it happens on corners, sparks emit a bit away from the wall. I thinks it can be easily fixed by changing wall's hitboxes a bit.
I think the game lacks some satisfying particles like small blood drops that fly from the zombies, bullet shells, dropped mags, parts of meat flying off the zombies or something alike. Just to make shooting feel really good. You can make blood pools, that you have now lie a bit longer and not disappear after few seconds. Also, make them darker, now the are very bright I think.
Making bullet projectiles a little bit more distinct from background can help seeing, what direction are they flying, that can make shooting more satisfying, because you will see how they hit enemies.
For quality of life, maybe it would be nice to have autoreload, it's sometimes frustrating to click and understand that you didn't reload. However, if you don't want to have autoreload, you can add a clicking sound for empty mag and also it's cool to have shot sounds for nearly empty mag, like it is in CS GO. It's just for understanding that you're almost empty. Sometimes it's hard to look at the ammo UI.
And yeah, about ammo UI. I thought about adding bullet counter near the crosshair, for example a half-transparent circle, similar to what is in Half-Life 2.
Probably, you can add a slight screen shake when shooting for more impact. Also, this way you can make weapons more distinct from one another and make the player understand that some weapon shoots heavy rounds, shoots slow/fast or deals more damage or less.