I'm not saying the whole age gap thing is bad per se. Lots of people have similar age gaps, and still have fulfilling relationships. My only gripe is that he knew her when she was nine...In my case, if I saw her as a child, I don't think I could see her romantically later on. Those memories of her as a child would still be present in my mind, at the time he probably saw her as a little sister or simply a little girl he saved. But hey, if he can forget all about how she was a little kid when they met, go for it, I guess.
I didn't mind the harem, because I thought the author would be able to handle it okay in the long run. But the power scaling in this is just completely stupid. He literally said he has recovered his body from when he was an immortal, and he got drugged?? How does that happen? So a mortal drug can knock out an immorta'ls body? He still has trouble with people who aren't supposed to be on his level, and just in general the plot is being forced forward or used solely for sex scenes.
What you have to understand is that the world he comes from is low tier compared to this one. to get a better understanding read "I’m An Evil God manga" in that story the mc travels from different worlds gaining skills, but one thing he realize is that when he try's to use skills from low teir worlds they barely did anything in stronger worlds.
Yang Kai hasn't followed a master in a few years, maybe this guy will change that. He's not evil, but he isn't necessarily good, just like Yang Kai, he does what he can to achieve his goals. Maybe he'll teach him a few things to help him survive in these new parts.