As I said before, Russia is more than capable of disabling all critical infrastructure in Ukraine. If their main objective was "annihilation", they would've targeted dams, bridges, fuel stations, food processing factories loading and unloading facilities in ports and train stations, water and gas distribution facilities, heating facilities, critical elements of the power grid, all sorts of communication towers, regional and central decision making centers, sewer water treatement facilities, food and medicine depots, etc. They can even justify it by claiming that Zelensky has plans to eventually mobilize the entire population (though they don't need to given all those sanctions). This could easily be more devastating to ukraine than several tactical nukes. The fact they haven't destroyed even some of the targets I mentioned means they feel no real pressure on them. Russia appears to have some sort of a plan (or plans) in this war. The strikes on powerplants were probably a punishment to Ukraine for being "naughty" (for lack of a better word).
Sorry for the long rant.