Shooting with the offhand
When I went through a concealed carry course, they played a Massad Ayoob video on shooting methods, and he demonstrated a technique for offhand shooting which looked interesting, so I gave it a try.
Basically, reverse foot position from the normal Weaver. Ayoob tucks his dominant hand up to his chest, as though favoring a wounded arm; he claims this helps his balance. I didn't find that this part made much difference. For martial artists, body is positioned as for a reverse punch, and this works just fine for balance, strength, etc.
What was very different was that he held the gun rotated with a wrist angle similar to what would be used on the handlebar of a cruiser bike, so that the sights angled across to the dominant eye. Recoil motion is up at a 45 degree or so angle to the strong side, cross body, but it isn't too bad. Sights do line up more rapidly this way than they do if they are held in the normal upright position, and the pistol doesn't have to be pulled across the body to the dominant eye.
First shot accuracy using this technique was noticeably better. However, follow-up shot tended to track diagonally up and to the right for me.
Clearing the (not ambi) thumb safety of a GSR using only the left hand is something that takes at least a little practice to do smoothly.
One other thing that I'd recommend practicing (with an empty chamber and no mag) is racking the slide using something other than the strong hand. Marines taught me to use anything from a between the thighs squeeze to the rear sight on a belt, shelf edge, etc. If you are shooting offhand, you may be down to one hand. Practicing this with a live magazine could be pretty dangerous, so I can't really recommend that, but empty it's a good drill.
Same can and arguably should be done with an unloaded pump shotgun, for the same reasons.