The best beef/fat blend for burgers is 80/20. However, beef tallow is not the best for flavor. So a trick is to get 90/10 beef, then blend in bacon grease.
It not only has a much better flavor, but is loaded with the “fifth” taste, savory, aka “umami”. It is why bacon is so loved, because only a few foods have a LOT of savory in them, and bacon is one of them.
Other things that can be added to the meat before cooking (not all at once) are liquid steak seasoning, liquid smoke, Worcestershire sauce (which also has savory), Tabasco or Louisiana hot sauce, or Hawaiian-style (with pineapple juice) teriyaki sauce.
And after cooking, but still on the skillet or grill, put a pat of butter on each patty and rub it around until it melts.
other things that will add umami are: tomato paste, pureed anchovies, pureed shitake/bella mushrooms, a little soy sauce.
i was thinking of grinding some bacon and adding it into the burgers.