Great methodical work & you should not feel discouraged
Now the problems appear to be limited to 'lost motion' & I believe you will improve matters even more.
I would suggest the following:
The replacement spring should have ground flat ends. Admittedly, not always easy to achieve but hugely significant to ensure that the spring is seated properly, as well as removing stress raisers (incidentally, cars that have 'square cut' suspension springs because they are cheaper to produce, have them break more than the ground flat sort) Same principle applies to cable outers cut down prior to having ferrules fitted. Which reminds me that even the cable outer ferrules can lose motion by deforming if they are not the flat sort. We're talking "nat's" here (well I am!) but it all stacks up.
The whole set up is new so will need to bed in. I cannot remember what pads you are using, but I know that having used sintered both on & off track, I'll never go back to any other. A while back I tried with good OEM pads & they were awful. The pads themselves always be fit from being linished on their backs to remove any spongey paint, but I just mention that
The actuator itself will also need bedding in, but if the materials/hardness are not as OEM, then there will be some lost motion there. I've fell foul of remade clutch centres deforming as a spring, causing drag, but they spring back to size! Difficult to diagnose but a revelation when I did......
The cable outer needs to be as short as possible. Almost too short!
Cable adjusters need to be the strong, stout (M7?) type & the lock nut tightened up.
Even the radius of the inner cable where it is clamped has an effect through needing pulling out when the brake is actuated. I usually apply mole grips top & bottom to squige the radii tighter.
One last thing that you shouldn't need to do on a road brake, but I do on hydraulic race set ups is to replace the static pad adjust screw with shims/washers.
I hope that helps