The point that I think you are missing, Cinders, and I was trying to make, is that I don't believe it is a case of Russia "taking tanks out of storage and sending them to Ukraine" but more a case of "Russia taking tanks out of storage, rebuilding them and then sending them to Ukraine". There is a difference and it takes time to rebuild an armoured fighting vehicle which probably explains why Russia has struggled to replace it's lost tanks and other armoured vehicles. It has been said that many of the Russian tanks in "storage" are stored outside; if this is correct it is easy to understand why they need rebuilding before they can be used again.
I wouldn't have thought it likely that President Putin would resort to nuclear weapons because he knows the consequences if he does. It is pointless worrying about something that is not likely to happen.