Giganto seems more like the big boss of this arc, beating him is the primary goal of this arc, he's the strongest, and there's literally a timer counting down to his arrival, if the league get Giran and defeat Giganto before facing Destro then they have no reason to stay and fight, they'll just teleport out of there
The difference between liberation army and the Yakuza is that Horikoshi isn't using the liberation armies full force so I don't think he'll be done with them this arc, also the top members don't equal the strongest members, they just have a high status, Destro most likely has some strong pro heroes on his side