According to the raw intel gained from radar, he could tell each force there had at least hundreds of millions of spaceships. And there were a huge number of flagships, meaning there were lots of generals and capable soldiers out there.
He wanted these, and on top of that, he didn't want to risk having such a force growing in the heart of this sector. If that ever happened, and they got wind of what he was doing, then the consequences would be quite severe.
So he had to make sure this force would be dealt with early on, before moving away, reaping all the loot he could from there, and attacking a few more gatherings while heading to the other two gatherings appearing on his radar.
His Soulers and Reapers, couldn't get deep intel about the ongoing battles too far from him. So he had to expect the appearance of such threatening gatherings that he didn't get yet.