A USB microscope is going to be overkill for failing eyesight. It's too much magnification, the field of coverage is too tiny, and they're jittery as heck. I have one, but the only thing I really use it for is for imaging details too tiny for my flatbed scanner, i.e., closeups of embedded silk threads in 1st issue revenue silk papers.
A good illuminated magnifier will do more for you.
Here is a compilation thread of various magnification devices I have tried for different purposes:
https://goscf.com/t/45998Hopefully there's some useful information for you.