Using old Kanji to learn new Kanji

This might not directly address your issue, but I would recommend giving the Keisei Semantic-Phonetic Composition script a try.

It adds for example to the lessons of 花 and 化 the following list:

This not only shows you that there are other Kanji with this phonetic component, but it also tells you if they share the same on’yomi reading (which in this case they do).