Black flash doesn't increase the power 2.5 times.
A punch with black flash equals the power of your punch to the power of 2.5
In mathematical terms: Punch ^ 2.5
NOT Punch×2.5
Have you ever though of using hamon? I feel like it’s an upgraded version of sun breathing since it basically does everything it does but also heals faster and you could use it as a way to enhanced chi training and maybe a way to fight the spirit. Idk just a thought