I wouldn’t say that Aegon ignored the Band of Nine. IIRC, WOIAF says that Jaehaerys went to war using plans that Aegon had drawn up before his death in the event he had to deal with them. If that’s the case, it implies that he had some criteria that he was using to decide when it was worth a war outside of Westeros, but whatever threshold he was using wasn’t met till after his death. It might have been that Aegon viewed the Blackfyres as a spent force after the Fourth Rebellion and he didn’t