the best ending this thing could get would be one where they DON'T end up together, DON'T do a timeskip to when she is 18 and where they stay as friends while isshiki gets engaged to a woman his age
Exactly, preferably he cross paths with Akari again, he properly apologizes to her for abandoning her in her time of need, they make amends, regain familiarity, and rekindle the flame of attraction and emotional bond they once shared.
Fumi dates and marries Akari, they have a kid together, and Ayano, now properly cured of her stupid, irresponsible older man crush, visits and plays with their child from time to time. The End.