They did that already, at relatively little cost. But doing so did necessitate that they also open a front in the North, at least to prevent their moves in the South from being countered by reinforcements coming from the North.I think that this force could’ve been effective, had the planners conceptualized and provided a coherent, focused, and achievable strategy.
This medieval, taking of cities was beyond stupid, and the multi-axes attacks on who-knows-what will perplex military historians ‘til infinity.
Simply advancing from Crimea to the eastern and western borders and somewhat north along the Dnieper would’ve been strategically logical, quickly achievable and easily defensible. It would’ve established a hundreds kilometer long front line that would require any responding forces to expose themselves to Russian air interdiction and artillery and MLRS fire while drawing them away from the both the capital and the geographic center of the country.
Also, inevitably there are a number of cities in the Donbass Region, such as Mariupol and the Severodonetsk Metropolitan Area that they must take.
The big mistake that the Russians made in the first week of the war, especially in the North, was undertaking cavalier charges of armoured vehicles with infantry, without any extensive probing, without recon extensive with fire, and well ahead of logistics, and not combining the arms of artillery and close air support with such advances. That was hubristic recklessness due to completely underestimating the capabilities of the Ukrainian forces, instead of adopting the mentality of caution and overestimating them. The Russians knew very well that the Ukrainians possessed effective anti armour weapons, drones, and decent surface to air defences against low flying aircraft, yet they undertook such a reckless charge, not applying the cautious and highly tried and tested and known to work methodology of the 2nd Chechen War.