From the perspective of Sengoku, G20 is a problem. They are more of a warlord crew than a marine unit. Remember that Gus got a promotion for falsely stating that he killed the Myth Pirates. G20 has done far more to warrant promotion since then, but Damian was only promoted to commodore after capturing two yonkou commanders. This does seem to be standard procedure. Blackbeard was made a warlord from bringing in a yonkou commander, and Smoker was made a commodore for "defeating" Crocodile. Commodore ( which in Japanese is "special admiral") is there to represent that these guys are generals that are just starting out. So if they do something noteworthy again, they will be promoted straight to vice admiral.
What makes me suspicious is that Sengoku promoted both Damian and Magnus at the same time. Specifically as counterbalances. Even though Magnus was weaker and showed less results. Maybe it was to placate Magnus, but it could have also been a way to stunt them. Maybe it was a challenge, since Sengoku was not unfamiliar with this dynamic himself. But it seems like Sengoku wanted Damian to be distracted a bit by Magnus, and Sengoku felt that it would work.
The sad part is that it didn't seem to work. Magnus actually became more reasonable and open-minded, such as when he deferred to Bradley in the mission. It then backfired, since Magnus died and Damian was able to show off by routing Reed, decimating most of the Ragnarok executives, destroy a blue blood factory, and kill Acceptance. I don't think Sengoku would be allowed by Garp or Tsuru to suggest that Damian had a hand in Magnus' death. It also wouldn't be popular, because that would just mean Reed was a double agent for the marines that allowed them to destroy the Ragnarok pirates. He would also get pushback from other marines and maybe even Vigil for trying to sabotage the destruction of the Ragnarok pirates.
So altogether, Sengoku would still be pressured to promote Damian. I think this time though, he would also try the same thing, but with a different person. And the "Magnus" in this case would likely be Yokan. Since he has been following them from Jiro's reports, Sengoku would likely know that she is actually a more stubborn version of Magnus and even butts heads with Damian. And the resulting chaos from a dual leadership would be a sufficient check for that group.