Welcome to the board. It sounds like you have a major project on your hands. Correct me if I’m wrong, but I thought that the Traeger pellet grills have three sittings: High (450), Medium (325), and smoke (200-220) degrees. 200-220 degrees settings will definitely qualifies it as indirect grilling, and even 325, since it does not have direct flames under the food. From what I read about this grill, it does nothing but indirect cooking (since the whole cavity acts as an oven and without direct flames).
As far as what to cook, I would recommend huge cuts of beef since your space is limited and you’re serving so many people. Roast beef will be your best bet. Use large cuts of beef such as Round (eye of round is the most tender), they yield so much meat, and they take little room on the grill. Set your grill to medium setting (325 degrees) and roast them until internal temp reaches 125-130 degrees. Let the roasts rest for 10-15 min, and thin slice them very thinly across the grains. If you have a favorite beef rub, apply it to the roasts prior to cooking also a beer mop will work well with beef. I hope this gives you some ideas. Let us know how it turns out.