Rather than doing a bunch of aim warmup something that helps me is warming up my fingers, hands, wrists and elbows, which helps me feel more comfortable while aiming and is beneficial because it can help prevent carpal tunnel.
Also, I would recommend getting something like a stress ball or similar and squeeze it until you get in game, it helps keep you calm before games, and keeps your grip and fingers warmed up from your last match.
Finally, if you play a really long session I recommend icing your wrist and fingers for a little bit just to make sure you don't injure yourself.