Suddenly drop one day to 1% on charger / Low watt charging on usb-c

I think your just running into a design limitation in this series (2016 onward have this failing)

Unlike the older Retina and Unibody models this series CPU’s & GPU’s can pull more power than the charger can supply! So your batteries power is also drawn and you are running your system hard your system can run into a low power event! As there will be a point the battery can’t back fill the needed power the charger is unable to supply.

I’ve see people who are doing video rendering on these system encounter this power short fall to the point they just can’t get the job done! The best they can do is make sure they have the systems battery fully charged using one port on one side for the charger and then using the other side with an external USB-C battery pack (as large as they can get) and the external SSD RAID drive they are working from. As the vid takes hours to render they setup the system inside a refrigerator! So the system is running as cool as they can get it (to get the most processing out of the system they can) and let it rip!