Egg, not a solution to your arc issue but here are a range of lines that are animated by one piece of @keyframes code.
You need to look at the perimeter of each but basically I tuned the CSS to produce an arc half the length of the circles.
Therefore for the other shapes, their Perimeters will not show as half an arc.
The point I am making is performing the animation directly on a Xara shapes with a line and its attributes set within Xara is a better experience than having to deal with the arcanery of SVG internals.
SVG-CSS - Animated Lines.xar
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