I have for you another piece of the crumbly meatloaf solution. Ever notice how very lean ground beef tends to crumble, and not hold together? Make sure that the ground meat has at about 30% fat in it. The fat holds the lean meat tissue together.
About the comment about frozen meat holding extra water; The water was always there, in the muscle cells. When the meat is frozen, the water expands and tears the cell walls, freeing up the water to flow. The starchy crumbs will absorb and hold that water in the loaf, or you could simply add a little flour to do the same thing.
I find that unless you make specialized meat loaves as i do, the primary flavors are cooked onion, salt, well done ground beef/pork, and maybe a little garlic. Added milk and crumbs add another layer of sweetness to the meatloaf, and soften the texture. Egg adds no flavor, but helps bind the structure together, and hold in moisture. Fat renders out, and carries away fat soluble flavors. Tne topping adds a contrasting flavor to the sometimes bland loaf.
Additions that enhance flavor include onion, garlic, sage, salt, black pepper. After that, you can further incorporate other flavors, such as Southwestern/Tex-Mex, by adding cumin, chili powder, peppers, coriander, chopped mushroom, etc.
You can make it Middle Eastern by using ground lam/goat/venison with currie powder, cumin, cinnamon, etc, topping with a Tzaziki Sauce.
Fro something more Italian, go with oregano, basil, minced carrot, marjoram, and Parmisan or Asiago cheese.
For french flavors, add celery, carrot, onion, and top with a nice Sauce Supreme, or an Espanole.
I sometimes make a chicken meatloaf flavored with finely chopped celery, onion, soy sauce, 5 spice powder, garlic, and topped with sweet and sour sauce.
Too much filler, i.e. breadcrumbs, cracker crumbs, etc., will make the loaf too soft, and mushy. Too lean ground meat will make it crumbly and dry. Too much fat will make it greasy. The loaf should read about 165' F. in the middle when done. Overcooking will dry it out. Under cooking will be dangerous.
You can tailor the meatloaf however you want, as long as there is sufficient meat, and the flavors compliment each other.
Seeeeeeya; Chief Longwind of the North