I'd suggest checking manga forums or dedicated Bleach fan sites. They often have detailed breakdowns and recommendations on where to pick up after a specific part. Sometimes, the author or official sources might provide cues as well.
Also, the character development in the fanfiction could be very different. In the original Bleach, Kenpachi is mainly about becoming stronger in terms of his combat skills. However, in this fanfiction, his development might be more centered around his relationship with Ichigo. He might learn about things like love, compromise and self - reflection which are not really part of his original character arc in the main story. And for Ichigo, he may have to deal with a whole new set of emotions and challenges that are related to this relationship rather than just his duties as a Soul Reaper.