I used to play a similar game name Silent storm which got aiming and injuries system nearly as you describle: go for the head make blind reduce vision and aim; shoot the hand mean lower aim, take more ap to do stuff, damage or drop off weapon, made gernade on hand explore; leg damage reduce moving ability. So this should be like heavy injuries only be treated out of combat, while the game progress there some unlock tech or skill can help treat heavy injuries in combat like character skill, peak, tech to allow treating, equipment heal fatal wound,...
But this system is complex, too hard for newbie but can be easily abuse by veteran.