Me too except I just use rye malt instead of the bread and without making any dough with it. The malt is cheap and easy to get in UK thanks to home brewing suppliers whereas rye bread is super expensive.

My only challenge is controlling the gassiness - it’s so vigorous that the moment I even slightly open the cap the whole thing fizzes up like crazy - opening it normally would result in a kvass fountain shooting up like 30cm. :)