Putin screwed himself and his country with this invasion. No matter what Russia does now in Ukraine, it's going to blowback negatively...
If Russia succeeds in conquering Ukraine, they'll need to hold it through any resistance, which will trigger PTSD of the disastrous Afghanistan invasion and occupation.
If Russia doesn't succeed in conquering Ukraine, it will have to retreat with its tail between its legs back into Russia...that will also show how weak of a leader Putin is, which will only embolden his opposition and Navalny.
If Russia assassinates Yelensky, they create a martyr and galvanize not only the Ukrainian resistance, but global resistance because targeting the leaders of nations is a big no-no in international warfare. It also opens the door to terror attacks and assassinations inside Russia...so it won't be the opposition being tossed out of windows...it'll be Putin's allies walking around with big targets on their backs. And Russians don't really care about collateral damage.
If Russia doesn't assassinate Yelensky, it has to contend with a media-savvy leader who undermines Russian control merely by his presence.
By inviting Belarus into this, Russia is showing its very, very weak hand...the Belorussians aren't going to swing this war of occupation to Russia.
Russia also weakens its leverage over Crimea and Donbass if it's unable to occupy Kyiv and Kharkiv...after all, if Russia can't manage to capture either city, it certainly can't maintain its current occupation.