As far as I can see, Russia has three possible responses.
1. Nuke a Ukrainian city.
2. Launch an all-out conventional war.
3. Continue more or less what they have been doing, just upping the intensity or more targeting civilians or something, to make a "statement".
Of those, #3 changes very little. #2 Putin won't like, because there (I trust) there are limits to the number of casualties he can absorb without triggering domestic unrest, and because it would reveal the "special military operation" to be an actual war, and not (so far) a success.
#1... If I understand correctly, Russian force doctrine calls for using their nukes when they have lost 20% of their delivery mechanisms, or when their homeland is under immediate threat. Losing 40% (perhaps an overestimate) of their strategic bombers isn't quite at the threshold, but it's getting into the neighborhood.
Problem is, Putin is trying to take Ukraine. Nuking it - even one city - diminishes the value of what Putin wants to take.
So option 1 is... I wish it were more clear that Putin will not do that. I still think it to be less likely than not, but it is far less clear-cut than I would wish.