Why would they intentionally hold back from achieving their objectives as swiftly as possible? They went all in at the beginning trying to take Kyiv thinking it would be a walk in the park and got kicked back. The more needless deaths the lower the moral in Russia so why not dominate swiftly and save the lives of your own people? They tried but could not and still struggle to take a few km here and there.
Just speculating, and from what others have said, the calculation always has to take into account what other countries will do as a response, not just Ukraine. Just like Ukraine has to take into account what the response would be if they somehow could kill Putin. Or what Iran's and the Muslim world's response would be if The Supreme Leader in Iran would be assassinated. I think Russia doesn't want the US involved even more overtly. They don't want WWIII. I mean I think Russia has held back on doing assassinations of the Ukrainan leadership until now, surely they could do that if they wanted to? At the same time I'm sure they have had miscalculations and fails, but they're supposed to have many contingency plans in place.
