25/75 or 20/80 don't go any leaner than that. Screaming hot steel griddle or cast iron....do it outside unless you have top notch ventilation over your range. Grilled burgers can be decent especially over hot coals but don't come close to a good burger cooked in its own grease. Propane grills, best results you'll achieve are vastly underwhelming mediocrity. Acceptable for a large crowd or a weeknight pinch, but not gonna be top-notch. The key is getting that nice flavorful browning that gives you flavor, but not a bitter burnt char. Much easier achieved on a flat surface than a grill due to the surface area contacting conductive heat surface.
Heavy salt and pepper immediately before you cook.. Throw in on the griddle, give it a press down to make sure you have maximum contact with the meat and the heat. Then don't touch it for about 90 seconds until you have good browning, flip once, cheese it, cook until its done. I'm a rare steak guy all the way but my burgers to medium well/well, the texture of ground meat that the proteins haven't set is disgusting. Thus my dislike of venison burgers, because at that point they are dried out cardboard unless you mix in other animal fat...at that point you don't have a deer burger, so what's the point? You can temp them, I go by visual. When it starts to contract just a little, pull them and they'll keep cooking some more.
Don't get fancy, don't mix all kinds of garbage in and turn it into a meatloaf/sausage patty. It's a burger. Meat, salt, pepper, that's it. Don't overwork your meat, it messes with the protein structures and makes them tough. Do the bare minimum to get it into a patty.
Simple potato bun, the burger is the star of the show here, not your neighborhood baker and his silly brioche. Toast the bun in the grease after your done cooking for another flavor/texture boost.
Lettuce, tomato, onion, mustard, and ketchup. Bacon and fried egg are acceptable add-ons when you really want to get at it. Mayo is a mortal sin against humanity.