Luffy's scar aches at the mention of Akainu and Oda made Dragon and his goons state over a dozen of times who their true foes are.
Their foes are the Celestial Dragons. Now I wonder, who are their main military assets? Right, the admirals, they serve directly under them. The moment Luffy laid hands on a Tenryuubito, they dispatched an admiral. You yourself posted the scan where it was said that the Tenryuubitos position is only guaranteed due to the admirals being their shield.
We can say that:
1) Their main goal surely isn't to fight fodder Tenryuubito because that's not going to change
2) They aim more are dismantling the heads of the WG, but they themselves were confirmed not to know the full picture, which is Imu (an immortal ancient king) sitting at the throne and will soon figure out that Imu is the arch enemy of Joyboy/Nika, so they understand that this is not their scope.
And since Luffy will fight Imu, the strawhats will fight the Gorosei. So the revos are either left with the Holy Knights, the admirals or both. As for Dragon vs. Garling, nothing calls for that. Dragon will neither be degraded to fighting Garling, nor some other Holy Knight scrub.