Crumple foil into balls for a trivet and oil the slow cooker lightly.
Stir breadcrumbs and milk in a large bowl; let sit 5 minutes.
Whisk in eggs, ketchup, Worcestershire, onion, garlic, parsley, salt, pepper, and paprika.
Add ground beef and mix gently with your hands until just combined.
Shape into a tight loaf, about 8 inches long, without overworking.
Set the foil trivet in the cooker and place the loaf on top.
Whisk ketchup, brown sugar, mustard, and vinegar to make the glaze.
Brush the loaf with half the glaze.
Cook covered on Low 5 hours or High 3–4 hours, to 160°F internal.
Brush with remaining glaze and cover 15 minutes to set.
Lift out, rest 10 minutes, then slice and serve.