I just realized while I was playing a game that my mousing hand tilts in a little bit. Some times this causes inaccuracy when I need a quick up or down stroke. I am currently playing games with a logitech g5.
The way some if not most people play is not the way they use their mouse normally on the desktop and posture isn't part of anything to do with how you use the mouse unless your desk and chair are just terrible. First of all your G5 is a palm grip mouse and the sensor is off-set. So if you move up and down your kinda moving in an odd way at least with a centered sensor or with a sensor towards the front center or front and left, it'll come out more normal. And just because it tilts in doesn't really mean it changes anything you just gotta understand it's not facing your position it's sorta like you believe you need your mouse parallel to the image being displayed because you control the character so it would be like as if your inside the game rather than controlling a character as if it were a puppet or machine.
Now the palm grip of the G5 does not allow you to move down. Seems like an odd thing to mention but the way it's used if your high sensitivity from reading what you said you try and move down when the mouse is already touching your palm. From the way you play sounds like you fingertip and or claw grip were by you hold it with the fingertips and you sorta leave a space so when you aim up(fingers go forward) and come back down(curl back) it comes out fine but when you move down your pressing up against your palm the only way to properly aim down is by slightly moving the arm down.
Something along the lines of
this. Except with a higher sensitivity but if you notice when he needs to look down he just brings down his arm that's pretty much a limitation of the claw grip.
I imagine that ideally when I mouse my hand should be aligned to my forearm; parallel to side of my keyboard.
Not really most play like that for the sake of playing like that but you can play with say your arm bent in or away from you or whatever way you want, sorta like
this. Like I said before you play as if it's you inside the game for the fact of logical reasoning but sometimes you need to decouple that habit and find a way to play differently to better suit you though I'd say in particular to a high sensitivity were mistakes punish you more than low at times.