Because brisket is a very large cut of meat, trying to estimate the overall cooking time is quite difficult. If you are planning on having 20 guests to dinner, I would highly suggest cooking the brisket ahead of time and keeping in the the refrigerator until game day and then reheat it starting several hours before the game.
Another option would be to start the cook about 30 hours before the game, allow it to cook overnight with the intention of the brisket finishing about 6 hours before guests arrive. You can keep the brisket warm by wrapping it in foil, wrapping it in a couple of large towels or a blanket and then allowing it to rest in a picnic cooler (no ice of course) until time to eat.
Racks of ribs tend to be fairly uniform in size, so it is much easier to estimate the cooking time. One popular method is the 3-2-1 method (although there is much dissention on this) that calls for cooking at 225F for 3 hours, wrapping the ribs in butcher paper or aluminum foil and cooking for another 2 hours (perhaps at a higher temperature such as 250-275F) and removing the foil and placing the ribs back on the grill to dry out the surface of the ribs. You want the final temperature to be 203F +/- as measured by an instant read thermometer. However, the overall 6 hours can vary significantly. Thus, plan on the ribs finishing at least an hour before guests arrive. You can keep them warm in a picnic cooler as noted for the brisked.
Please remember that the temperature shown on the Traeger controller might not be the actual temperature inside the cooking chamber. Also, if you are cooking multiple racks of ribs inside the cooking chamber, they might not all cook at the same rate. Measure the cooking chamber temperatures independently from the controller using a 3rd party thermometer or probe. Check the internal temperature of each rack of ribs periodically with an instant read thermometer and move ribs around as needed to insure uniform cooking.
Although reading recipes and watching YouTube videos is helpful, nothing beats actual experience. If this is your first cook, I would predict the success of cooking everything at one time and having it all ready when the guests arrive is quite low. Give yourself a chance to succeed by getting as much done before game day as possible. Otherwise, you are going to be stressed when the guests arrived. That can ruin a great party. I wish you well!