We are trying to jump through the hoops to sell soda bottled in TN. I've gotta get a Regulated Services Permit and the 1mil limited liability insurance.
The permit is quite tedious. We have to have our kitchen inspected. Take a class on food services procedures for handling safety.
Fun stuff
Do you not let it sit and carbonate for a few days before you chill it? It is also possible that the temperature that you two are allowing the yeast to carbonate the soda is not the same.
Actually although they seem thicker they are actually weak. I stored some of my soda that I was premix dispensing at an event for later in one of these type bottles and my soda expanded the bottle.
I would not suggest trying to carbonate after you add the syrup. instead I'd carbonate 4 gal of Water then jumper over 1 gal of water In port to In port.