Danielius, what are three main things to keep in mind when you try to cook the best burger?
Practice, practice, practice.
Practice aside I'd recommend to focus on three things:
1) Meat composition
Try grinding your own meat using different cuts of beef: shortrib, ribeye, chuck, brisket, etc. Cook the patties alone, taste the individually and try to find your sweet spot. Try different meats - mix ground and chopped salmon, turkey, pork (pork is the most underrated meat IMO), you get the idea. But the key point here is to taste the patties on their own to get the idea of their individual taste.
2) Meat cooking
Buy an immersion circulator and it will save you tons of time. However, if, for some unimaginable reason, you want to skip this step... Try cooking the patties without oil, with a splash of oil. Heck, try deep frying them. When I don't have my circulator, I cook the burgers on high, flipping every 15-20sec (this makes them cook more evenly). Again, cook the patties on their own to get the idea how they turn out. The point of this is to get the idea how your stove and cooking method relates to the final texture of the meat.
Try all the cheeses. Processed ones, hard ones, goat, sheep, mild, sharp, blue, green. Try making your own blend of cheeses (my personal favourite is provolone + gruyere, but it will be a different one for you, I promise). Also, if you really want to up your cheese game, learn to make your own processed cheese. No thank you needed.
When you have mastered all three of them (or get bored with experiments):
Try differen buns - potato rolls, pretzel buns, brioche (overrated), make your own buns!
Different sauces (for example: wasabi kewpie mayo for salmon burgers)
And only then I'd suggest you start adding condiments to your burger (be careful not to overstack your burger). Try wakame sallad (and all the others), chutneys, guac, caramelised onions, black garlic, literally anything you can think of - try it.