If the Gorosei were actually top tiers (who are all stronger than the Admirals, as you guys seem to be arguing) then Kizaru isn’t even necessary, or the battleships.
In a world where top tier Gorosei is true, the sensible plan would be for three of them together, which is a force that would annihilate every Yonko crew, to go to Vegapunk. Don’t need to tell any Admirals (better not, keep the entire thing hush hush). And there’s still two other Gorosei back in Mariejois just in case.
What has actually happened is that events have forced their hand.
The world at large is completely chaotic
They don’t want Vegapunk to discover any more of the Void Century lore
They want control of the Seraphim away from him for good
They have sent one Gorosei, but kept the others back in safety. They have taken Kizaru, probably because he’s the only one available. Fleet Admiral Sakazuki is the man in charge and can’t leave, Fujitora isn’t trustworthy and Ryokugyu was off hunting Weevil.
End of the day, mission goes wrong, worst case scenerio, they can still say “well, we lost Saturn and Kizaru, but there’s still us four, two Admirals and Sakazuki.” It‘s a necessary risk they‘ve taken to get rid of Vegapunk.