Shouldn't it be relatively easy for slow head Xiang to break through to the ninth rank by using traditional methods now? He almost made it with the best quality pill, so even if there was some relapse, his starting position would be a whole lot easier than ever before. Like if he made to 90% now, so even if he fell to 40% due to being interrupted, it would still be extremely easy compared to starting from 0%. In fact the recovery from 40% to 90% would probably be a walk in a park, relatively speaking, while the remaining 10% could be difficult, but still nothing compared to a random joe or jane starting from 0% with no prior experience. Xiang might be the slowest man in the West, but 400 years of cultivation ought to be enough.
But then again, there's not that much logic in this story, so who knows.