I think the original move to Kyiv was to try to get 3elensky to quickly come to the table. But once that didn't happen, they kept a force there to hold up forces in Kyiv while the real fighting was being started in Donbas. They had roughly 40k troops there at the time. There is no way they would have realistically been able to storm a city of 3 million with that number of troops.
As far as the Donbas and southern Ukraine, they mostly see the Russians as liberators, not invaders. They have little desire of going back to the Kyiv regime at this point.