Are the Ukrainians actually having troops enter Russia? Or are they just taking advantage that their advances have brought Russian logistic’s hubs in Russian territory within conventional artillery range and continuing to blast at Putin’s weak link. Ukrainians promised not to use our high tech longer range artillery to attack Russian territory, but now they are in range of the kind of conventional tube artillery both sides possess and which Putin had used heavily against them (while his cold war ammo stockpiles had lasted.) Putin could pull his ammo dumps back farther to avoid this, counting on the Uke’s not wanting to risk expanding things further. However, IIRC Putin’s rail net’s major N/S lines are just east of the border and he can’t move them. Since his best logistics are by rail serially cutting those lines—even if they can keep being patched—will impact most supplies locally and further south.
So you are suggesting that Ukes could target these rail lines while only using conventional artillery, cause minimal collateral damage, and avoid a tactical nuclear penalty?