I feel this is one of those times, where calling things by their English translation would have been more useful. "Yonko" as everyone knows means 4 Emperors. In fact, this was probably inspired by the Roman Empire's year of the 4 Emperors
https://en.wikipedia.org/wiki/Year_of_the_Four_Emperors
But people don't really internalize what it means to be an Emperor when they call it Yonko. What comes to mind when talking Emperors? It's vastness of their reach. It's land. Wealth. Material possessions. Armies. Alliances.
Shanks and Mihawk were both Top Tiers when they were fighting. Any increase after that would only be incremental. It takes Shanks years/decades to become an Emperor, not because of strength, but because of all the politics, sailing, alliances b.s. The reason Blackbeard became a Yonko so fast is because he usurped Whitebeard's empire by taking out Marco & co. Shanks didn't have this luxury. He started from scratch.