G' Day Uncle D... Your brake operating lever appears to be over centre with the brake applied. So you will not achieve the maximum available force to apply the brake. The wheel is on the rear limit for chain adjustment, and consequently the hub brake lever at rest is set too far forward.
You need to adjust the operating lever on the cam spindle so that at rest it points further to the back of the machine with the linings just kissing the drum. With the brake fully locked the lever and cable should never pass over that 90 degree angle. Any stretch in the outer cable must be overcome first before the inner can exert any real force, so this is another source of wasted pedal movement and a spongy feel as the spiral wound outer acts as a compression spring.
The secret of good stopping is to achieve full lining contact, with both shoes initially contacting the drum at the same time so follow Musky's advice and see if the fixed pivot is adjustable to centre the shoes.
The cable run is really suspect.... it looks too short and also wound through the frame will give an additional friction loss. A nice straighter cable run is what you need. It has been run like that to make it reach. Plus S/A movement could have stretched the outer as detailed above. However, the front lever is not set up correctly. Moving the cross shaft on the pedal splines to allow the front cable lever to point backwards to be almost parallel with the sloping frame tube and with a correctly set rear hub lever will increase the mechanical advantage of the system. It will also effectively lengthen the inner cable and allow the cable adjuster to be backed off so the drum lever can be set to point further backwards. As set now, the more you apply the brake, the less force you have, as again the front arm and cable are on the wrong side of the magic 90 degrees.
Get the brake internals set up properly to achieve minimum drum/lining clearance.
With a bit of luck some cable dimensions will be along soon from other forum members, so you can see if it is the correct cable, just badly fitted. Or simply the wrong one. I reckon it should be a bit longer and run outside of the frame loop, and be supported by a P clip on the vacant torque stay mounting.
My first post of 2019, so Happy New Year to you all, and may a small drip of oil from your breather pipes be your only problem.
All the best for whatever 2019 brings.
Swarfy.