I’d say the secrets for complex burgers are balance and compatibility.
Absolutely. The only thing I would add is that if we're talking about the "best burger", the burger has to be the primary player.
I love me some monster burgers. Throw a fried egg and bacon on two patties with some Texas toast for a bun. I had a great burger up in Vermont that was topped with mac and cheese and maple syrup. And I'm not afraid on gimmicks. But none of those really taste like a great "burger" because there's just too much going on. It's like a high-budget special-effects driven movie. Super fun to watch/eat, but also pretty far from cinematic/culinary art.
In the other direction, I have no shame in downing an inordinate number of White Castle sliders at once. And there's an ancient diner near me with amazing greaseball burgers. But I scarf those down the same way I take in The Brain That Wouldn't Die or Them! Super enjoyable garbage that I will defend forever, but not a substitute for real food.