When Is the best time to do all this tests/analysis, before bottling the beverage product or once it’s done as FG?
Your thoughts are highly appreciated. 
Testing should be done on samples taken from completed product.
Once completed, before bottling, then a few samples after bottling.Usually first, middle and last. For random.
Quick tests Brix, acidity as Citric etc Inline or once stirring procedure adhered to.
The preservative etc as Benzoic acid etc could be done before bottling, but usually the proof of addition is enough. A shame a whole batch gets destroyed, because it fails basic testing requirements.
Tea is still, Carbonated is trickier and the carbonation is part of the physical testing undertaken.