Playing at 1080p is fine and will actually help reduce input lag.
Most important step is to disable in- game V-sync. Then set frame limit to custom with a value of 300. These changes got my input lag from about 16 down to 6.
The change felt so significant that it took me a couple weeks to adapt to the newfound responsiveness of my mouse.
More great recommendations for settings here:
Reducing input lag won’t automatically improve your aim, however it will remove an impediment that will allow your aim skill to progress if you put in the practice.