You can attack a country with nuclear weapons, provided you use only conventional ones, then threaten to escalate to nuclear if they do that. Nuclear weapons are the ultimate threat which would ensure mutual severe damage if not destruction if used, therefore nobody uses them first unless they're completely nuts, or they're cornered. Putin is a criminal but he's far from being crazy, and as for now is surely also far from being cornered. Surgical attacks in
Russia with conventional weapons would undermine his powers and create enough public disapproval to facilitate a coup from within, but should be done with extreme care and up to a certain point in order not to trigger a nuclear response.
Sadly, even using conventional weapons, the number of deaths would be huge; it is entirely possible that Putin would sacrifice millions of innocents sending them to the front line before giving up, also because when dictators give up they usually die shortly after.
Black/Azov Sea aside, they're not touching the area where the power resides, which is usually needed to weaken the leader image. Last bombing in Belgorod is probably just an error, but in any case it accomplishes nothing aside giving more fuel for Russian propaganda.