I'm not trying to argue with you, but I do want to bring attention to one thing.
If we follow the logic that even the main character himself stated as well as tang wei, these stones should make him even stronger. By now, in the story, he's found 2 stones, so absorbing the second one should at least put him at peak houtain. I'm simply pointing out the plot holes I'm seeing,
This is assuming two things:
1. That qi stones are all made equal. We don't really know this; the second qi stone may have just been less potent than the first.
2. That going from middle-Houtian to peak-Houtian takes the same amount of effort as going from mortal to middle-Houtian. It most likely isn't.
Take Tang Ruo for example. They enter early-Houtian a few days (or weeks) after entering the sect. Then they enter middle-Houtian right before the expo. This was over the course of several months. Now a 2.5-year time-skip happens, and they've only reached late-Houtian.
We know Tang Ruo never stopped training. So that could only mean the time, training, and power necessary to ascend to the next stage will take longer every "level up," so to speak.
That's it. Personally, it makes sense to me. Though, you likely know more about cultivation stories than me, and so if that runs contradictory to other xianxia tales, then that's fair criticism. Have a good day.