Honestly, the shichibukai's strength is something which oda simply mishandled... Because as the series went on their existence made less and less sense. And not only because they were actively harming the world, oda simply didn't develop them in a way which actually justifies them as one of the 3 great powers. We have had several iterations of shichibukai:
1.- Mihawk, jinbe, hancock, moria, crocodile, kuma, doflamingo.
2.- Mihawk, jinbe, hancock, moria, blackbeard, kuma, doflamingo.
3.- Mihawk, buggy, hancock, weevil, law, kuma, doflamingo.
4.- Mihawk, buggy, hancock, weevil, kuma
Now.... at what point does it make a shred of sense for any iteration of this group to be a force comparable to either the marines or just one yonko crew? Based on what we have seen so far the marines are a massive organization with extremely powerful warriors. The fleet admiral, admirals are powerful by top yonko commander standards at least. And the VAs are numerous and skilled warriors. I would go as far as making the case that vice admirals are basically the marine's backbone and their stronger members are basically the marine's response to yonko commanders (though not necessarily top yonko commanders).
And on the other hand you have the yonko. yonko crews have a basic structure so far, though each one has its own quirks: At the top a yonko that towers over its crewmembers. Then usually top 3 executives and a handful or two of executives who are very powerful in general. And then there are other minions who are usually affiliated crews. And their strength can vary from yonko commander to pretty weak. Overall each yonko has massive organizations behind them.
Based on what we saw at marineford... We kinda need to factor in even hype for any one shichibukai to be admiral level. Well, BB eventually became a yonko but I doubt he had that level at marineford. He might have that power now though. From the shichibukai in general... Honestly the only original shichibukai that looks like he might be yonko level is maybe mihawk, assuming he can still hold a candle to shanks. And even then we have to consider shanks probably does not rank very highly among the yonko. IMO kaido and big mom are more powerful than him.. Hancock by hype alone appears to be extremely powerful as well. And then... Moria and crocodile do not appear particularly powerful. Moria was "fired" from his position on account of his weakness even. Doflamingo in turn appears to be about as strong as maybe the weakest of a yonko's top 3. And even that is optimistic. I don't think doflamingo would win against cracker or vista at least. Kuma seems pretty powerful but its hard to place him anywhere tbh. No real reference of where he ranks. All we know is that Iva can hold his own against him and even that is not much of a reference. jinbe is pretty strong but IMO he ranks at about mid tier yonko executives, just about beneath yonko's top 3.
Overall, the shichibukai appear to be weaker than a single yonko's main executives. Granted that the 7 in any iteration are an extremely powerful group but they just can't be single handedly as strong as even one yonko crew, let alone all 4 of them. And its utterly absurd that they are a force comparable to the marines, which actually manage to contain the yonko's power and influence of 4 yonko within the new world. The problem the marines have in this regard is that they can't actually gather all their firepower to take out the yonko as that would leave massive amounts of territory unprotected.
