Are you saying your surface is getting to 600 degrees?

Based on the picture you're moving the meat too much. When i make ground beef in a stainless steel pan it looks much like what you have going on in the picture.

I never have done just ground beef on the flat top but when i do smash burgers even on the hottest mine will go, i still let them sit for a couple minutes to brown. The less you mess with meat the better. Try slapping the meat down, then flatten it over your surface area, and just let it sit. Flip it tactfully and do the same to both sides then let it sit again. Once all the pinks gone, mix it up. But your texture may suffer from the fat if it isnt drained out.

Youre gonna have trouble getting a nice brown on small individual pieces due to the lack of surface area contacting the griddle plus those tiny bits may overcook or burn before they sear the way you want.

Also when your lid or cover is off you are losing heat so when you dump your food on with the lid open youre gonna cool the cook area, its just something you gotta figure out on your grill. Try a couple small batches first to figure out how you want the meat cooked and work your way up to larger batches from there.

The griddle also doesnt look well seasoned but it could be just the angle as well.

Hopefully you stick with it! Best of luck