I received my Ravpower Turbo 20000mAh powerbank and did some testing. Here are the results for 3 different powerbanks I tested using a type-c voltage/amp tester while playing BoTW. Also tested the OG power adapter to see the power draw.
5 Year old Anker 9000mAh Powerbank
Cable: USB-A to USB-C
Power draw: ~7W
Xiaomi Gen2 20000mAh Powerbank
Cable: USB-A to USB-C
Power draw: ~7W
Ravpower Turbo 20100mAh Powerbank
Cable: USB-A to USB-C
Power draw: ~7.5W
Ravpower Turbo 20100mAh Powerbank
Cable: USB-C to USB-C
Power draw: ~10W
Original Nintendo Switch Power Adapter
Power draw: ~22W
So in conclusion, the most value for money powerbank will be the Xiaomi one, It can charge the switch while playing but expect slower charging speeds. Best way is to make sure your switch batt is at 100% then plug in the Xiaomi powerbank it will drain the powerbank first.
If you want faster charging go for the Ravpower but it is not cheap and with the new battery shipping regulation that require you to pay more to ship battery, might not be worth it.
If you are those Elite then get Omnicharge powerbank that can plug in the original AC adapter, it cost like USD$299