On Lei Shen, you have time for 1 combustion before the transition. So there is no need to rush it really. Wait for WUSH, even if you have to wait 45 secs and your Combustion will be ready for phase 2. In that case, Cha-Ye is a bonus on top of WUSH if it procs together.
You combust twice before the first transition. So delaying combustion for 45 seconds probably isn't ideal. I guess I'll just keep combusting with cha/pot/jade spirit/springs and pray for Wush.