Typically this would require something to be gained from engaging in nuclear war. If you bring out the nukes then you are pretty much asking to be utterly destroyed. Sure you may get some of your enemies in the meantime, but you won't survive to see them dead. Using nukes in Ukraine is just a quick way to turn every other country in the world against you.

I think it is far more likely that Russia is waiting for a change of leadership in America that is friendly to their cause. In the meantime they are digging fortifications and stocking up for a future offensive to retake Kursk and expand their holdings in Ukraine when they lose access to American munitions.