Great photos Anh!
Im using similar size hot plate and I have to say that soldering leds on bare pcb is little risky. You should not exceed 1.0-1.5 deg./sec temperature raise for modern leds, and this impossible with such powerfull hot plate.
But, you can fix two problems (fast temperature raise and pcb fixing) with one trick. If you are using copper or brass pill you can solder it to the pcb same time when you solder leds to pcb. Extra mass will prevent fast temperature raise, thermal pass between pill to solder and solder to pcb give extra several degrees difference so this lower solder layer flows before leds and you can easily determine when process is ready.