OK jmez, we'll start with your biggest issues, other than maintaining control over this event, - warm holding.
You must have with you an accurate instant-read digital thermometer and a pocket full of alcohol swabs for sanitizing the probe. I advise you to record the temperatures of all potentially hazardous menu items before and during service for your own protection (liability). You should also have a packages of disposable sanitizing hand wipes. They are handy to have when someone drops the tongs in the dirt.
You are going to sear warm SV cooked steaks that have been warmed just prior to searing for an hour to 125F - 130F in 2 borrowed propane turkey fryer water baths, with thermometers for near constant monitoring along with long handled spoons for some tumbling action to ensure even heating.
Prior to immediately before warming all steaks must be held under constant refrigeration or on ice in insulated containers. No "room temp" ever!
You will have two discrete streams of steaks by doneness from vacuum packaging, labeling, SV cooking, ice bath chilling and refrigerated holding, warming, to decanting into pans, to searing, to service. 4 steaks to a bag will be convenient.
Hopefully at least 2 of the 6 "BBQs" will be preheated very hot grills and big enough to accommodate 8 or 10 steaks at a time. That's all you need with 2 competent people searing using 18" tongs and protective mitts. A typical single service line will just about match the timing of your production output. Arrange the grills so any wind is at the cooks' backs. Get a couple of large shakers of steak seasoning to apply during searing.
Steaks will be served ASAP after searing. That why you are doing them in smallish batches which provide your time control. When all the steaks are on the grill its time to start turning the first ones after seasoning them. When all are turned it's time to place those first ones in the pans and send for service. Place steaks in labeled (Medium / Medium-Rare) new disposable aluminum pans from which they will be served (more tongs).
Be prepared for several Well-Done requests. They can be seared first and kept warm (150F) in a spare BBQ at low heat before service. A few people may feel entitled to "seconds". Have a plan for it and stick to it.
Lastly, how do you propose to serve your 80 guests?
Buffet service (Which i recommend unless you do this often and are just playing with me.) You will need about six 6' tables. 2 for the actual service line, draped if possible, which has plates, wrapped cutlery, bread, sides, and then the served steaks last;
1 table for beverages apart from service area,
I table for desserts separate from others.
2 tables away from the service area for supplies and support work.
Several large lined waste cans.
Or there's Table service, individually or Family Style with bowls and platters. - get's complicated and labour intensive offering sides and doneness choices.
Next time you might want to consider hiring someone who does these events for a living.