i hear you on the bullshit added ingredients as far as eggs and breadcrumbs and other stupid shit mixed into the meat
BUT, i'm not opposed to trying dry seasonings outside of salt and pepper....lots of good mixtures of flavors to be had out there in a shaker or mixed up on your own. but if you do a smashburger correctly and get that good crust going, it will over power the flavor of almost any seasonings you hit it with, especially if you cook the thin sliced onions into the meat on the flip.
Pretty sure it's a repost but I just follow the directions on the back of a lipton French onion soup mix... the one with breadcrumbs, mayo, etc etc. This is for charcoal grilled thicc burgers, smashburgers are salt pepper and on steel.