Anne,
I use a meatloaf pan for my meatloaf.

Yes, I agree with you when you have a lot of meat in your sauce it needs a longer cook time.

I simmer it all afternoon! But there’s a lot of meat in my sauce (chuck and meatballs).
One more question – do you cook the meatloaf on a cookie sheet or in a meatloaf pan?

Anne,
Yes, you should only simmer your sauce for a short time.
I used to simmer it for an hour until we went to Italy. I asked many of the chefs how long they simmered their sauce and most said 15 minutes.
You only need to simmer the sauce until the olive oil rises to the top.
It tastes fresher the shorter you simmer the sauce.
I know many people simmer their sauces a lot longer but you need to constantly stir the sauce so that it doesn’t burn.
How long do you simmer your sauce?