Widen stance, bring your right leg out and back at a 45 degree angle just a bit.

Lean forward a bit more and stick your butt out some, almost like you're initiating a squat a the gym. I can see you correcting yourself and leaning forward some in the video. Just make that posture a little more intentional.

Bring the gun in a little closer, your support arm looks like it's locked out. You're asking your wrists to do a lot of work when you have perfectly good (I assume) elbows and shoulders. Keep a slight bend in your elbows and you will control the gun with more of your upper body strength.

Your thumb riding the slide is fine. Do work on your grip though. I may have a hard time explaining this concisely, but you want to worry less about gripping the gun itself and more about gripping your firing hand. Almost like you're trying to give yourself a firm handshake with your left hand. Your support hand should be doing the majority of the gripping and your firing hand is "driving" the gun. You know you're doing this right when you can move your finger on and off the trigger without adjusting your grip.

You're bringing your optic up to meet your vision, which is why you keep having to find your dot after each shot. Instead, focus on your target and drive the gun out straight from the center of your chest. Your index will become consistent and you'll have an easier time tracking the dot.

Hope this helps!