You have done some really good fault finding and done all the right things.
Voltage drop is a very good indicator, out of curiosity, What other commands have you tried? ( if any)
lf search
t5 detect
???
Have you tried using the following to help
https://forum.dangerousthings.com/t/handy-dandy-tips-and-tricks/13041/2?u=pilgrimsmaster
https://forum.dangerousthings.com/t/handy-dandy-tips-and-tricks/13041/3?u=pilgrimsmaster
Other than PM3 and iCopy, do you have access to any other LF readers / writers?
A flipper zero perchance?