Looks like you answered your own question.
Those are large kegs you're using (half barrel). Most homebrewers use 5 gallon corny kegs, and a few use Sanke sixtels, of similar size.
Obviously, a 5# CO2 tank is too small for your application.
Look into using larger CO2 tanks, 20# minimum I'd say, which is also about the maximum for easy portability, when serving on location, if that's your goal.
50# or larger CO2 tanks would be more economical. Good for carbonating the soda in your shop, but harder to move to events, or for exchanges or refills. You'd (almost) need 2 people to lift a 50# tank, same for half barrel (15.5 gallon) kegs.
Larger tanks can be used in your shop, but those will likely require truck delivery for filling, pumping liquid CO2.