Pan temperature is everything, obviously!!! I've tried to perfect smash burgers for years, much like trying to master a tricky golf swing, believe me. I've used cast iron, stainless steel, flat-top griddles... you name it, I've tried teh lot!!!
It's absolutely about the meat hitting a screaming hot surface. Don't use lean meat, ever! You need a decent fat content, 80/20 is the absolute minimum you should be considering. The fat renders quickly, helps teh crisping, and holds all the flavour. And smash it once, very firm, right when it hits the pan, then leave it alone... no fiddling!!! That's how you get your proper Maillard! Don't worry about protein denaturation too much; just get teh pan to 500F-plus! It's not rocket science, but people make it wierdly complicated, don't they?!