Today I changed my 20year old front brakelines.
Before I had good brakes, but they just didn't feel very firm. You had to push the pedal all in.
With the new Goodridge lines the pedal feels much harder from the begining.
So if you want beter pedal feel, change those old brakelines.

Changed the rears today. Pretty easy job.
Now my handbrake locks the wheels with ease.
Feels like an S13 handbrake.
Handbrake initiation no problem.

What does the handbrake have to do with the brake lines?

Or do you have a power handbrake?

The handbrake uses the same lines/braking fluid as the "pedal rear brakes", or not?

Mux means this:

If do not have a power handbrake the handbrake is controlled by a mechanical cable (similar to those on your bike) from the handbrake lever to the rear wheels where I guess there is a small brake drum inside the brake disc that functions as the handbrake (right?).

I believe that it's only a disk brake with no drumbrake inside.
What do you mean by power handbrake?

The handbrake mechanism is operated with a metal cable, so what kind of brakelines you use should not make any difference unless you have some kind of hydraulic handbrake mechanism (custom built).
Consider this: maybe the fluid-change and proper bleeding caused 90% of the effects you are describing, cause I havent noticed such dramatic improvements from fitting goodridge lines at all.

I dont know why your handbrake is working better though, I believe it mechanically pushes the piston out a little, so it should work no better or worse than it did before, unless you either rebuilt the caliper or fitted new discs/pads at the same time.

Your handbrake works with cables
they have nothing to do with your steelbrakelines.

Noticing a lot of change when installing the steel brake lines probably is caused by the old lines being very warn out. If your old lines are already of relatively good quality there wont be much difference, remember that standard brake lines are also breaded on the inside. Steel brake lines for the most part, are just cool

But the better working of the handbrake is interesting. Fresh pads have made the biggest difference for me, but handbrake still sucks I think it has to do with the fact the pistons don't have to come out as far and its easier to jank the handbrake.
I actually read something about adjusting the handbrake to make sure that the pistons only have to push out a little.

I had bleeded my brakes 4 times before. With not much results. Also changed the pads but not much change in the pedal feel again.
I think that my brakelines were just old.
Why the handbrake works beter, I don't know it just does.