I have found a trick to make the spaghetti lines more bearable :wink: by placing two lines on top of each other.
One line is routable and has borders. The other line is non routable and has no borders, only a solid colour.
In the line style file it looks like this:

highway=x [0x060 road_speed=1 road_class=1 resolution 20]

In the overlays file:

0x060: 0x06, 0x10106

In the typ file 0x06 is for instance the line width=3 border=1

[_line]
Type=0x06
UseOrientation=N
LineWidth=3
BorderWidth=1
Xpm=“0 0 2 0”
“1 c #FFFFFF
“2 c #393000
String1=0x04,street
String2=0x03,straat
String3=0x02,Straße
ExtendedLabels=Y
FontStyle=SmallFont
CustomColor=No
[end]

0x10106 can be a line width=3 border=0 or a 3 pixels wide bitmap:

[_line]
Type=0x10106
UseOrientation=N
LineWidth=3
Xpm=“0 0 1 0”
“1 c #FFFFFF
ExtendedLabels=Y
FontStyle=NoLabel (invisible)
CustomColor=No
[end]

The result is looking much better on my old Etrex device:

without overlay

with overlay 0x10106

Of course on a newer generation GPS like the Dakota it looks more pretty, but the differences are not too big: