"using lean ground beef";
You can use leaner than 80/20 if you add milk or broth, stock or wine, as with meat balls.
The liquid makes a softer mix, and eggs make it stiffer. The bread crumbs have to be sataurated well.
Eggs have dense protein molecules. That is why the whites have to be separated and whipped to make them light for a merangue', or soufle', and cream of tartar, alum, baking powder or soda added.
Also makes fluffier waffles, omlettes, and fritatta's.
Skim milk is fine to use.
I lighten up recipe's with it. For macaroni and cheese with broccoli and chicken breast, I make a butter and flour rouix, but use skim milk. It thickens up nicely.
My favorite special meat loaf is the Two Fat Lady's hedgehog, with juniper berries, and mushrooms.
Betty Crocker had an old recipe with broccoli, ham, and cheese rolled up inside