His new idea to try cultivating with heavenly lightning means that the benefit that he gained from the fight is not limited to the short-term increase in his cultivation. It means that he might be able to replicate the increase in his cultivation by directly utilizing heavenly lightning for cultivation.
This is the second reason why he can't regret participating in the fight against the Sun King. Instead of regret, he wishes he could do it again.
He is so happy with his situation that he wishes he could try out his idea for cultivation immediately. Unfortunately, there are no trees around for him to acquire life force for the Wood Vessel Sigil technique, and there is no lightning in this world for his Lightning Vessel Sigil technique.
