I suggest shortening them so they are the same length, or using a long but short piece. I think shorter ones look better.

I have suggested four color combinations. I would prefer suggestion 4. It suits the surface best, in my opinion.
Every player has their own taste.
As for the decorative walls, I was extremely satisfied with your implementation of my suggestion. So it's up to you. 😎

Was thinking about to "include" the lines into the overall road geometry, not as texture, more like an indepent object, to control its colors.

If y see that it doesn't make much trouble, y may do it.

It will increase overall geometry, since the lines will be a geometry apart of the main road.

This is alot of work, and will take some time.