What's the efficiency of doing this over being able to change the laptop battery? I've done it a few times for my M1 Macbook Air and feel like it can't be much better than 50%-75% but it would be nice to know.

I would not take that wager. Just because it has a USB-C port doesn't necessarily mean it has support for anything other than basic USB device support.

Huh? How else would you charge this device?

There are laptops with USB C that still have a separate power port to charge.

HP Elitebook G3 for one is not happy with power from a phone charger, idk don't have a powerbank to test.