If it's too vinegary, you can shorten that first ferment time. The length of fermentation can really vary based on the ambient temperature of the house, but I usually start taste-testing it around 2 weeks.
And we've done second ferments in the fridge no problem
It takes a little longer for the flavors to develop, but if you're not drinking it *that* quickly then it really doesn't hurt (and way less chance it will explode
)
My friend used to just chop up and throw in whatever fruit was in-season
You can also play around with some dry spices - Pumpkin Spice kombucha tastes remarkably like mulled cider.