Re: Strong single line on top layer
Might be wrong, but from the first look those gaps at the corners etc. I'd say you're underextruding (although I'm still new to 3D printing as well).
Pretty easy to test:
This typically won't result in a perfect 5cm distance.
Use the following formula to calculate your extrusion multiplier: multiplier = 50mm / (100mm - remainingdistance)
In my case I ended up with a remaining distance of 53mm, so the filament moved by 47mm rather than 50mm. Putting the numbers into the formula results in a factor 1.0638.
If your calculated value is outside the 0.9-1.1 range, there might be something wrong with your filament spool or extruder (like screws too tight or loose).
Open your slicer and in the filament settings there should be an option for the extrusion multiplier (defaulting to 1). Insert your calculated result, then try printing again.