I just don't buy that the Marines are doing all they are capable of, and I don't see why.
First off, this balance of power against the Yonkou is crap. They don't have the force to counter the Yonkou, they have the force to counterbalance a Yonkou. They had all 7 Shichibukai and the entire Marine Fleet at their command and disposal, and they could barely or could not match Whitebeard's wrath (depending on how you take Blackbeards crew showing up as part of the marine force). Either way, they would have been utterly fucked if Kaido had showed up as he intended to, or if Shanks decided to keep the war going instead of stopping it. They came close to utter destruction and were only spared by the good will of another Yonkou. This is supposed to be balance?
In the SBS, Oda revealed he had 2 other pirates that were candidates for replacing Kuzan and Akainu. Besides that, the conscription order Akainu sent out somehow conscripted the 2 admiral level fighters, meaning they somehow have leverage on them to join, but had not used it until now. Then we have Garp who is supposed to be Admiral level, but remains Vice Admiral for...reasons. Meaning, before the whitebeard war, they could have had 3 more Admirals on the frontlines. Now, if they have that many people at Admiral level, why not promote them to that level? Why have only 3? If they want a show of power, then they could have atleast a few more that genuinely deserve the title, and more personas of power.
Now, the Shichibukai. It's hard to talk about them because people simultaneously argue they are useful as a deterrent while also noting how utterly useless they are because of their betraying tendencies. Now, it's worth noting that literally every single one of the Shichibukai, past and present, has in some form subverted the WG, either ignoring or outright contradicting orders. So they aren't reliable as actual allies, but they are worth it for the appearance of power. Okay, but in that case, what they need to do is keep a few genuinely powerful ones, and then get a bunch of Buggy's. They don't know Buggy is actually weak, but what they need to do is find of bunch of pirates of middling strength, then propoganda the fuck out of them, and then they can say they have their own privateer fleet that feels formidable, but is ultimately controllable. Or, they could just seek out strong pirates that will actually be loyal, which they can do by providing incentives other than "We'll ignore your crimes" which obviously they don't care that much about. Something like maybe "We'll protect your children". Roger probably would have been tempted by something like that, given the shit that happened with Ace. Or, hell, just be honest and go "Listen, we're out to fuck the Yonko, and we're willing to let you ally with us to that purpose. Now, you may not like us, but if we get the Yonko, all their territory would be up for grabs, and as a Shichibukai, you can take your pick" That will give no small number of pirates a bunch of power boners.
So, I'm not seeing a good explanation here. If they want to keep a 'balance' between the Yonkou, they fail because they could barely stop one. If they want to eventually retake their territories, then they are really dithering about on that front, because they can appear a lot stronger than they are now if they just were willing to open up more positions in both the marines and the Shichibukai. There are drawbacks to the latter, but given how Whitebeard trolled through them the last time they decided to rustle his jimmies, I don't really understand what they are trying to do if they're not strengthening themselves to deal with the Yonko's.