Ukraine has technically already targeted Russian cities such as Belgorod and since Russia considers Crimea to be theirs, Sevastopol too, and if we're taking the piss, Kherson/Melitopol/Donetsk/Luhansk, Lol. But those are a grey line, however, Belgorod shouldn't be and even Crimea shouldn't be, but the retaliation has always been usual Russian stuff.
We should be giving Ukraine ATACMS so they can penetrate deeper behind enemy lines and yes, deeper into Russia, but I don't agree that they should use it for civilian infrastructure and there is very little strategic value in attacking civilian energy infrastructure like Russia is doing, it's doing absolutely fuck all to slow down Ukraine's military, it's doing nothing to hurt Ukraine's domestic support.
Even aside from a moral standpoint, it's simply a waste, if we're thinking tactically, the only reason to do it would be an attempt to break public support for the government who dragged them into war but I've absolutely zero confidence that happens in Russia of all places, they're clearly half of them terrified of their own government and the other half supporting them in the Nazi campaign.
Ukraine should get ATACMS to use deeper behind lines but if they use it on Russia, it would likely be (and should be) against military infrastructure (barracks, rail lines, oil depots, etc). But sadly, some countries are still too scared about giving Ukraine stronger stuff.
If they target Russia sadly it will unite people in Russia and they will want a war as they will spin it as a Nazi attack. Best thing to do is just keeping taking them down slowing in Ukraine and taking back land. This will hopefully open more eyes in Russia. Who knows may eventually lead to an uprising in Russia if Ukraine take out enough Russian weapons and trained soldiers. We are already seeing people question why they have to join this war and lack of weapons, pay etc.. The more they try to force people to go to war the more they expose how unprepared they are to even defend themselves from an uprising within.