Becoming a Yonko is about your influence over the New World in general.
Some people accomplish primarily through conquest of territories, such as Big Mom. Others, such as Kaido, accomplish it mostly through military force & attrition. Shanks and Luffy probably have the most balance of those two factors. You don't necessarily have to check all of those boxes to be a Yonko though.
There's also a pretty big implication in my eyes that your bounty largely has to do with longevity unless you are actively doing grand things. For example, Shanks is almost certainly stronger than Big Mom, is implied to have better commanders, was part of the PK's crew, AND has his fair share of territory from what we've been told. Yet, she is a bigger bounty than him. I think this is because she's been in the Yonko game for much longer. The WG noted that Shanks was by far the youngest of the original 4 and was only a Yonko for a few years.
This would explain why people like Luffy & Blackbeard, despite doing so many damn things, were given such comparatively low bounties when they first became Yonko.
It makes sense that Buggy's bounty wouldn't be as high or higher than Shanks' at this point.