I don't think that the decision to cross that river was driven by military strategy, but by political direction. They are under extreme pressure to make advances, and the generals involved in the planning likely felt that they had little choice other than to follow the orders of their high command, who are dealing with an unstable dictator. Hitler often overrode his generals, making the fateful decision to attack Stalingrad before reaching their major objective of the Caucasian oilfields during Operation Edelweiss. Putin had very definite ideas about what his military could achieve, if it invaded Ukraine, and he has been extremely frustrated by their lack of progress in living up to expectations. Crossing that river was key to making advances even though pontoon crossings in active war zones are extremely risky, especially with good satellite intelligence to spot and report ground activity. They were sitting ducks. Putin is playing with his toy soldiers, and the generals are likely being handed their military objectives under his direction.