I thought I'd chime in here with my own experience in kombucha brewing and the process of hard kombucha brewing. First - you should know that you do not need to use caffeinated tea to brew with. I have been using a mixture of both de-caffeinated green and black tea. Get a brand that does the de-caf process without using chemicals.
Second, in my experience you only need 1/2 cup sugar per gallon to get the same taste results. My guess is that if you use more sugar, you are just consuming more sugar with no souring benefits. I think typical kombucha recipes just copy eachother and figure that people prefer the sweet taste. But if you drink a lot of this stuff, it may not be wise to consume so much extra sugar that has not been processed by the yeast.
Third, the low ph in kombucha will stall almost any attempt to ferment hard kombucha. Yeast just can't handle it. It may work for 1 day then fizzle out. Typical DIY recipes will have you create a new sweet tea (sugar + green tea), then only add 4 cups of the sour kombucha tea to it, plus yeast. This in effect raises the PH to a level where many yeasts can handle. In my experience it is not that good tasting though. It comes out more like a sour wine. And in many cases it fails - probably due to the lack of nutrients for the yeast, as well as the dropping PH as the ferment progresses.
Currently I believe that to match the taste of commercial hard kombucha, what you really need to do is brew a batch of hard seltzer at say 10%abv then when finished, dilute with 50% sour kombucha to taste. Hard seltzer has its own challenges to brew but I think it will get you closer to a better tasting version of Hard Kombucha once mixed.
Just my two cents! I am still experimenting too.
