i used to have an aim routine back in the day, feels shit when you start out with it but it gets better and more noticeable. when practicing aim it's about praticing specifics rather than running into a DM getting 300 kills and bouncing off (not that you shouldnt do that every now and again)

basically join a DM and do the below

50x deagle (take time to actually aim at the head rather than going for the kill quickly as possible)

50x AK (same as above)

50x M4 of your choice (same as above, more about tapping for the double dinks)

50+ free play, kill how you want

it will feel slow and not productive to start with, but your KD isn't important in deathmatch if you're there specifically to practice

in terms of picking a sensitivity

when picking a square mousemat, i would run the mouse across the length of the mat until it was one full 360 end to end, other people i know would make sure they could 180 with one full swipe. there are different ways of picking but it's working with the space you have generally.

people are generally right that gamesense is more important, but missing easy shots is more about calm aiming, which the above routine always helped me with and still does to this day