Thx for the comments.
However, as this is alternate history, several butterflies must be considered, and countries which don't seem to have much of a chance can have greater control.
Although you might state that Korea can't extend beyond the Amur, I think that it's possible because other empires such as the Mongols and the Qing managed to do so, and the former had very little soldiers. The Rourans are just there, and really don't have definite boundaries, as they occupy deserts and steppes. I tried to make the Roman Empire and the Huns as accurate as possible, the Guptas and the minor empires are probably accurate, and the others are just guesswork.
Korea probably had the resources and the willpower to conquer China. When the Sui invaded with over 3 million soldiers, Goguryeo repelled them successfully, drowning most of them by breaking a dam. Of course, it was a defensive war, and they eventually fell to the Silla-Tang alliance, but the fact is that they were able to stand against such a formidable opponent. Because China was divided into several kingdoms in OTL 435 AD, I don't think that it would have been much for Goguryeo to effectively invade and conquer the area by utilizing their resources effectively if they had decided to turn their attention to conquering such a vast country.
If you have any objections after this, feel free to state them.