Side pull rear brake is weaker after changing the cable
It's hard to see what the surface of the brake pads looks like and how it is aligned with the rim, but I would look to see how flat the pad surfaces are and how flush they are with each side of the rim... when the brakes are pulled on.

If the pads have developed a concave profile where they used to fit on the rim, and don't fit there any more, it could explain reduced braking power. Replace the pads if they are no longer flush with the rim.

The black outer cables should have metal ferrules on the ends, in photos 2 &amp; 3 at least. They’ll help, but if they weren’t there before then it doesn’t explain the reduction in power you’ve had. I would also thread in that brake adjuster on the top brake arm, all the way down (back off the locknut first).