Istanbul was never a part of the Russian/Soviet empires. If Russia somehow annexed land as far as that city, history would be so different that I wouldn't even be sure if a Soviet Union as we know it would still exist.
While true . If the Russians pushed for it during the crimean wars or during ww 1 if they had their collective shit together . Sure..
If its ww 2 you need to have turkey involved unless the soviets have invaded turkey.
There are other issues with this whole idea, if things were that bad Greece is moving in.
So what if it's a Moscow aligned communist Greece .
Or Bulgaria swoops down.. Or Serbia moves over..
If you dismantled the ottoman empire and carved out spheres the Russians still wouldn't get it, the Greeks would.
So I'm I agree l, history would be vastly different.
But best shot, a free hand during the crimean wars and no one holding them back.. In that case it's konstaintinopl.. Not tsar grad or or othee whack names . Hell even the turks didn't change the name, they just call it what they do in Turkish which is Constantinople/Istanbul.
Now the Russians felt they were the 3rd Rome and a somewhat torch barer just like every other group since around 380/1480 ad .
Turks just moved in and set a byzantine shop with Islam.