They're designed to be flushed. You can install the kit and get it flushed if you're having issues. Whether you need flushing at this point depends on water quality where you are.
Dang, Terry, what's in the water that
YOU'VE been drinkin' & bathin' in?
(Or was that get up what you wore for Halloween? )
Flushing won't cause a leak unless the anti-lime agents uses are significantly more acidic than specified by the manufacturer. White vinegar is usually strong enough (and never too strong).
In extremely rare instances a heat exchanger can theoretically have a leak that is sealed over by lime deposits and begins leaking when de-limed, but if it's that far gone it needs replacing anyway. That's not too rare for embedded tankless coils in space heating boilers, but I've never heard of a clear instance of that happening with a tankless water heater.