Hitler certainly didn’t get pushed back in 1941!
300 quality tanks with well-trained crews would be significant. How well-trained the crews would be is certainly open to question. What’s the opposition in tank quality and crew quality? Likely not that good, all they probably have is quantity.
Surely the former Warsaw Pact nations have bunches of old Russian tanks, T55s, T-62s, & T-72s. Are those stores exhausted? They would make more sense to send. There would Ukes with experience in those vehicles. Also, the border nations would have a nice little business refurbishing them.
IK. The siege of Leningrad was broken June 22, 1944. The Russian winter was also a large factor in pushing Hitler back, but 300 tanks is not going to be the magic bullet for Zelensky but it will be more filthy lucre for the cabal.