A little fat doesn’t hurt. Browning them in a bit of oil helps render the fat, which can then be discarded.
I personally cook my meatballs most of the way, usually, when throwing them into the sauce, as when I make them, I’m usually making a Sunday gravy kind of thing where there is already a good bit of fat being rendered away from other meats. If I’m just making a plain tomato sauce with meatballs, I’m fine just browning and throwing them in without any sort of baking.
These aren’t related to meatballs but if you want to add extra flavor to your spaghetti then cut some velvetta cheese into small cubes (less than 1" on each side) and cook them in the spaghetti sauce. Also cube some garlic into tiny parts and mix that in the sauce too. That will make your sauce taste a lot better.
As for meatballs I don’t make them, but my mom does. She uses a mix of sausage and beef but I don’t know her recipe.
OOPS - here is a corrected recipe:
2 1/2 pounds ground beef
2 eggs (I use 1/2 cup of “liquid egg-like product”)
1 cup bread crumbs
1 tablespoon garlic powder (I use “pizza spice”)
2 tablespoons mixed Italian herbs
1 teaspoon salt
1 teaspoon pepper
1/4 cup grated Parmesan/Romano
1 tablespoon lemon juice
Mix, shape into 20 meatballs (around 2 1/2 ounces each), and bake for 20 minutes at 400 degrees.