First off, I don't think you should brew with decaf tea. Whats the ambient temperature? Was the tea completely cooled down before you dropped the SCOBY in? Did you add enough starter tea? As far as the plastic, just make sure its HDPE and you're good.
I was told by a commercial brewer that they use a BRIX meter to more or less tell how much sugar has been consumed by the culture. I found one on Amazon for about $25 and it has worked out great.
I'm not a pro or anything but I recently started kegging/force carb. my kombucha and that has changed the taste. Most commercial Kombucha brewers force carbonate after the Primary fermentation which IMO doesnt have that strong of a vinegary taste. My bottle conditioned Kombucha has always come...