most realistic scenario that you would consider to constitute a victory for Russia?
The two main objectives were announced at the beginning of the operation, and only their achievement can be considered a victory. At the same time, the form may be different.
As a result, there should be no breeding ground for the Bandera "strain" of Nazism in Europe (Canada will remain, but this is out of scope of current operation). This is not up for discussion at all. If there is even a square meter on which the Bandera infection can wait out and start spreading later, it is not a victory, but just another postponement, like in the late 1940s, through the fault of Khrushchev and his wife.
Besides, the situation requires détente with huge stockpiles of weapons and ammunition, accumulated by the USSR in the Western Military District in case of war with NATO and inherited by Ukraine during the collapse of the USSR. Ukrainians are hoarders, everybody who served in the Soviet Army knows that a Ukrainian petty officer is a blessing for the unit... He'll pick up anything ownerless that isn't nailed down as he walks past, and he'll always have plenty of things in his store that the unit isn't supposed to have. ;) Such huge stocks made many people's heads spin, their shelf life was not infinite, and they were not allowed to sell them on the side. It seemed to somebody in the West decided, that instead of allotting money for disposal one could try to turn all this splendor against the one who had mostly paid for it in the past. The second task of the operation is to eliminate these excesses, dangerous for the unstable Ukrainian consciousness. At the same time, it is desirable to make the overly inventive geopolitical "engineers" in the West bitterly regret about their "bright ideas".
It is too early to say how all this will be achieved. Now the most combat-ready and propaganda-motivated Ukrainian cadre units are being ground up (the captured Ukrainians believe that Belgorod and Rostov were taken by the Ukrainian army and that only in their area they got some particularly stubborn Russians and some Chechens in addition). Once the fortified areas created over eight years have been destroyed, it will be clear what to do next. But, IMHO, until the Russian troops meet with the Polish, the tasks cannot be considered even tentatively solved.