From your description I think you need to clean your charging slot 😛, a lot many phones are discarded because of this single issue when they can be saved so easily.  

 Charging must have worked. It's at 80% after booting 🤷‍♂️ 

Maybe the third Powerbank that I connected did the trick. Honestly don't know .. 

 "Sometimes it just takes a little extra power to spark things back to life! 🔋✨ Glad to hear it’s up and running! Who knows, maybe the power banks are secretly in cahoots! 😂" 

 next time press the power button to pause the boot for a few mins until battery gets above 3 or 4% 

 once i have had the same issue - problem was gone after letting the device charge before powering it. I was also terrified about this.