I never use any extra fat in mine personal choice, tongue master seasoning works for me.
If I run out and I usually do just salt and fresh black pepper keep the meat cold and mix to get a constancy that sticks to your hand (watch SRP on YT good advice) burgers and sausage are very subjective what you like may not suite your friends or customers.
Mine do not fall apart even with no seasoning (only salt and pepper) and only minced once, a nice slice of Stilton usually accompanies my burgers.