Middy
1000+ Head-Fier
Soldering that small is a pain even with a microscope. You need at least a temp control iron an a fine tip. Flux a tiny amount so none goes into the driver housing. You could do a j fish hook with the cable in the back of the driver to add a little wiggle room on strain relief. If the litz is of a smaller diameter than the standard cable. Strip off a little from the old cable and add to the end of the litz until the outer housing. That should allow the sheathing to grip the outer O ring and give a little more strain relief.
Just ideas to think about. Good luck
Just ideas to think about. Good luck