Luffy never starts with powerful attacks because his attacks actually damage him as well. If the enemy dodges then he just wasted some of his limited time. Luffy is a volatile fighter that adapts his playstyle to his opponents and wins in the long term. If he busts out strong moves that just end up missing then he increases his chances of losing.
Kizaru is a particularly slippery opponent with unknown devil fruit abilities. To top that off, defeating him was not really the purpose, but delaying him and pushing him away from his team without them taking damage. With that in mind, the best way to go about things is testing everything that would allow Luffy to catch up to Kizaru. When all of them did not work, he went all out and was forced to take Kizaru out with all his might in the end. Perhaps in a fight where Kizaru was looking to kill Luffy and time was not against him, Luffy would start an all out attack sooner, but would still test things out first.