It says plainly on the controller to make sure the battery is full enough to complete the update. It also says not to turn the drone off during an update. You should never try an update with a low battery. What you have now is probably a doorstop.
That's useful info, if you haven't used the Mavic again, you can also re-insert the card and switch on - and the file will often finalise and close correctly from the onboard buffer.
All I have is a M1P. I got it used for about $500 several years ago. I have 5 batteries for it as well. I did install the NLD software which removed the NFZ's and altitude restrictions. I do have an external RID so I can be legal (if I want...