Long, short story:
Am bad-to-average FPS gamer, looking to improve. Saw an OSU! gamer flinging its “shots” left and right and tried my best to mimick the experience in an FPS. And it worked significantly – all of my shots felt more “secure”, even with my aiming being rusty beyond belief.
And the “trick” to do this is simply leaving your “aiming hand” to aim (ONLY) while leaving your “non-aiming hand” to shoot and everything else. That’s it.
Well, I tried this idea with Planetside 2 (Which is a very “messy” game where it can easily use more than 10 hotkeys at a time) and it started to feel “natural” after 30 minutes - 1 hour of gaming. And can’t really give you a key mapping that will work for you “just like that” but try placing your “non-aiming” hand at the keyboard (right over your commonly used movement keys) and pay attention to where your finger(s) are and change your shooting hotkey to where one of your “non-occupied” fingers are. Then test it for a while, and see if it “feels right”. If it doesn’t, then switch to something else.
As for me, left shift felt more “natural” since I use WASD and my left hand (when relaxed) lands exactly to where the left shift key is.