The lines are actually drawing correctly, have a look at a side-view. You can improve the topo line “height smoothness” by mesh tessellation however this is faking topo data which is usually a bad practice for civils…
Getting to your issues; the flickering is due to the resulting viewport line thickness and antialiasing combined. You could improve it by increasing in the settings. I don’t think you’ll achieve what you’re after though. My advice is scale the line thickness with screen distance, do you really need to see that many thin lines? Think about major line scaling and hide minor lines at X distance away…
To get a true consistent topographical line thickness I think you will have to draw the lines as an overlay in the viewport (3d vector to 2d screen).