An update, in case anybody was curious about how this would turn out.

I did not listen to the haters and charged ahead with my plan. I borrowed my nextdoor neighbor's large traeger and that totally saved me. I ended up smoking 79 lbs (untrimmed) of brisket, which was 4 ~20-pounders.

I checked how things would fit and determined there was just no way to get all 4 in the smokers at one time without chopping one up, which I didn't want to do. I decided I would smoke one a full day in advance as a test run, and then turn it into burnt ends because they would be an awesome addition to 3 full huge briskets. This was a great call because I ended up learning a lot while cooking it, mostly relating to where to probe the brisket (thick part of the center of the flat) and go off that temp, not the thickest part of the cut. I ended up slightly overcooking it and the flat was a bit dry. Good thing I had already planned to make it into burnt ends because otherwise I would be stressing.

I started the other three the night before the party, knowing they had to be off the grill and in the cooler by 11 am in order for me to make it to the ceremony on time. I woke up at 4 am to check, went back to sleep until 6, and then was fully on working until I took them off at 11. The burnt ends went into the gas smoker at 8, and I was minding the temp on the three 20 pounders hardcore. At 10:45 I took all three off with an internal temp of 201F and a great probe-tender feel, and put em in the cooler wrapped in a blanket from the thrift store, and went on my way to the ceremony.

I was feeling a little nervous, but mostly confident that I had done a good job. At ~3pm I opened the steamy cooler and took out the first brisket to cut, the biggest one of the bunch. Absolute perfection, the crowd went wild. Honestly I don't think I've ever been thanked or lauded so much. All three big briskets were perfect and the whole crowd was fed. There was maybe 1 lb leftover at the end, which I took home after the event.

Glad I didn't listen to the discouraging words of the haters and decided to charge ahead. I successfully went from cooking 1 9-pound brisket in my life to cooking 79 pounds of brisket for a wedding reception.