It might be more of a cost than you think. Like he did say he had enough SP points to tame her. So that means that taming 2 or more people takes more points.
Still call BS. Even taking relitive levels into accout, he tamed that gang a few chapters back when they were multiple times his level. He was also able to tame the swordswoman in just a few minutes at nearly half his current level.
So while I could see it taking a bit to brainjack the strongest douche canoe, his two party members would have fallen in seconds.
The only reason he didn't is because the author/mangaka didn't want to repeat the same solution the MC just used against the gang just a few chapters back.
Instead, the strongest party will be off to either die to the floor 90 boss, or be captured by the "evil kid" that's sitting on the throne.