Im pretty sure the being inside him is beefing w the king, feels like its heavily alluded to that hes using Taehyun to get to the king. He can't do that if Taehyun dies, and Taehyun was going to kill himself. So he does seemingly have reason to wait. He's obviously not Taehyun and would wreack havoc before ever reaching the king. It wasn't a waste imo. It introduced us to the alter ego's actual wants and plans. I also don't think he necessarily got stronger. He seems to have the same strength, just grew mentally/in confidence from the experience.
So basically what I'm saying is that the alter ego seems to be waiting because he is stubborn! Both him and Taehyun are stubborn and they're not gonna let the other do what they want without interfering. Taehyun said he'd rather kill himself than achieve nothing and he brought the alter ego back by threatening that. The alter ego says he doesn't care about "this vessel" but he doesn't want to let Taehyun die because Taehyun is "alive for his goals" apparently
I feel like these last few chapters were kinda a waste. Like he got stronger for losing to his alter ego? I would prefer him to win in a stupid way than it still being a potential obstacle that’s literally inside him. Unlike beings like the nine tails where there was a seal. His alter LITERALLY has no reason to wait. It’s obvious he could prolly eat stronger without the MC, so what’s the point of it being like this.
That’s just my opinion though. I personally don’t see y this is the case but maybe it’ll have a good reason later