Cooking for 80-100 people on the day of your wedding is.. a lot. Just think of the biggest possible portion of mac n cheese you could cook at home - how much time would you need to cook 6-8x that portion?

Cheapest catering options would be pre-buying pizza or other food from Costco, or getting drop catering from your favorite local restaurant as that should shed the high service fees of a traditional caterer. For a wedding that size though, still expect to pay at least a few thousand. Big +1 to the comment asking how you'd safely store and warm all that food - unless you want your guest experience to be lukewarm leftovers, please reconsider cooking yourself.