They should have maintained more realistic, limited objectives from the start, preferably in the southeast, ideally securing the Donbass. Not only would this have allotted them far greater initiative with their initial offensive in an area where they already had strong local popular and military support, but it would have deprived the Ukrainians of both the propaganda value of an indiscriminate Russian invasion and the morale boost from the initial victories over a clumsily planned and executed bum rush from the Russians. Gobbling up Ukraine piecemeal from the start would have almost certainly resulted in less international support and less Ukrainian resolve.

Granted, going after Kyiv did tie up Ukrainian forces that otherwise might have been sent eastward. I'm a complete amateur when it comes to actual military tactics/operations/strategy, but the benefit of tying up Ukrainian reserves around Kyiv and the slim chance at being able to don't seem worth it, even if the actual attempt was handled relatively competently and with reasonable objectives. I suppose the best case scenario would have been Zelensky fleeing the city when the attack on Kyiv first started, possibly even fleeing the country. Fleeing the city would have been a major morale loss for Ukrainians, especially those in Kyiv, and fleeing the country would have been absolutely devastating to Ukrainian morale, albeit far less probable than Zelenskt fleeing to Live.