This game desperately needs basic work done.
Gun Swapping
Controls for this are really irritating, to the point where it honestly ruins the experience. I keep trying to use the EquipPrevious key to cycle through my guns and it will either do nothing, or it will change the gun I have held once or twice and then return to doing nothing. Ideally I would be able to load all the guns I have and then be able to press one key to swap them out for one another as the situation demands a different type of firepower. Not sure if this is the result of the EquipPrevious key cycling through empty inventory slots, or if it is the result of it working off a list of literally the thing you were previously holding, but in either case it needs reworking.
Drug Use
There need to be some key-binds dedicated to drugs. Currently to swap out what you're holding to use a drug and go back to what you were holding you have to go through six steps: you have to press R, then press the inventory number where that thing is automatically assigned, then press your Fire key, then you need to press R, then you need to press the inventory number that the gun you want to use is automatically assigned to, and finally press the Fire key.
This is way too much shit, especially on the fly with enemies chasing you. Ideally you want a single keybind that causes you to use the drug, and then returns your gun to your hand once the drug use animation ends. One key, one step. Simple.
Surrender
The animations can be rather long and tiresome to sit through, especially once escape is no longer possible. Having a key you can press to surrender to the enemy and skip to the end of the animation would be nice.
Testing/Firing Range
With the only way to experiment with the guns and drugs being in the levels they appear in, it is difficult to get a feel for their characteristics and how best they might be used on different enemies. There needs to be some out-of-level space for trying out all the things the game offers.
EquipPrevious key literally swaps between two weapons. When you use it, the weapon you just had becomes the weapon you had last equipped. Deliberately swap between two weapons you like between waves to set it as a backup weapon. Spamming it won't give you anything but those two guns.
Use the number keys to quickly access the rest of your guns. Pressing the number key multiple times cycles through weapons of the same type, from lowest to highest. You can manually sort them by dropping weapons and picking them back up to slot them in as the lowest priority.
Same goes for drugs. I think you can press Q to quickly get back to a gun if you need to use drugs in a fight, but the number keys are probably more reliable. Obviously, emergency drugs go on the bottom, healing goes above, manually sort by dropping and recovering... and go over the carry limit because the game didn't quite expect people to do that. You can't just assign hotkeys to each drug individually, that would be insane.
You hit R. Left-click. Hit the number key of your weapon type to grab a gun again. Three inputs. I don't know why you think you need to take out another drug before switching back to your weapon, or why you're counting firing your weapon again as part of the process to use a drug.
These are not new concepts. This inventory system dates back to Doom and has been a core principle since. It is not broken, you are not used to it.
Adding a button to skip one of the primary selling points would be lul. You got hit and knocked down, now the wave gets to crowd around you. Sounds like you're being punished for getting hit.
A firing range would be... fun-ish, I guess, but trial and error is literally a core part of the challenge here. The more information is presented straight-up about weapon and enemy performance, the shorter this game becomes. There does not, in fact, NEED to be one.