> 2. I can plug RPi to it and I can plugin a charger in 2i. When the main power cuts off, It'll instantly switch over to battery with 0 downtime. I think this is called passthrough charging)

This alone is a killer feature that’s far less common than I expected before I started searching for it. Most power banks cut off output while they’re charging, and many require physically re-plugging the device to start powering it again.

I ended up acquiring a PiJuice for this functionality alone.

https://github.com/PiSupply/PiJuice