@comrade ever wondered, why would anyone bother training anything if it is only about haki
Did Garp waste his time crushing mountains to dust, or on the battleship bags
As Rayleigh said only way make CoC stronger specifically is to grow stronger yourself, basically a mix of CoA & CoO, skill (whether martial art swordsmanship or a DF specific style of fighting) and physical strength
If you are strong your haki is strong, and vice versa, haki transcends all in that sense, and haki itself grows strong by being strong