How would you do that in sync with removing cargo, the cargo which probably is a cargo container, and is just disconnected, you can't just suddenly move ten tonnes of water onboard the airship, it takes time to fill up.

A cargo container of water? You're literally throwing your hands up at the smallest issues and pretending you hit mount everest.

Not to mention there's no requirement the weight has to stay the same. Inflating and deflating the balloon controls the buoyancy.