National Review reports Ukraine Present Zelensky says fighting in Russia is inevitable and attacks on Moscow are absolutely fair. Zelensky is of course correct.
Russia has been bombing the crap out of Kyiv; why isn't Ukraine justified in doing the same to Moscow? Of course it is justified.
Doing so won't be easy and probably won't happen often. Every couple of weeks should be often enough to make the war real to Muscovites.
Ukraine can shoot drones and cruise missiles at Moscow until the capital's air defenses become too formidable. Then switch off to other military and civilian targets within a few hundred miles of Ukraine. Russia is too huge to afford the same dense protection everywhere there are useful targets.
It is unlikely Ukraine can defeat much larger Russia on the battlefield in any mano a mano sense. If they are persistent, however, they can wear out Russian willingness to continue to expend lives and money with no end in sight.
This happened in Afghanistan to the Soviets, as Russia was then called. More recently it happened to the US. The Afghans wouldn't give up and eventually occupiers went home. I believe it is the most likely end to the Ukraine war.