My only concern is that sugar is an avoid for me, and I don't want to waste a whole bunch of agave (expensive!) on a recipe that isn't "tried and true." I may experiment with this, using regular sugar, and feed it to my kids, and then play around with making an agave version for myself. Or maybe try using sugar for "stage one" and then agave in "stage 2."
Or stick with beet kvass for my probiotics and water, iced tea, and seltzer as beverages.