Yes variable outlines is not supported by standard SVG, so when you apply the effect in Inkscape they will be converted to path.
But if you are comfortable with Inkscape, one advice would be - don’t add any variable width, just keep it as a stroke and when you are in Synfig change the width parameters.
You can also use raster images in Synfig, not necessary that they must be vector.
Also Synfig it self doesn’t follow all SVG standards (for vector shapes), for example.
You can see the above svg, White frame is a single object, the inner vertex points are looped and outer are also looped and they fill the shape between the inner vertices and outer vertices, anything like this is not possible at all in Synfig, there is workaround for this (you can create a shape and set it’s blend method to alpha over and you will have that hold). But it doesn’t support it natively. I understand that Synfig is for animation so it’s not necessary for it to follow SVG standards as there would be no need to export for SVG.
But just considering the above example animating I wanted to scale the inner vertex points larger than the outer vertex points (i.e inner vertex are now outer vertex). You can’t just do that in Synfig, you have to change the blend method of both object (put the opposite blend methods). That also adds a little more work, it doesn’t seem a problem for my example, but some times there might be no way.
(I don’t know if I am using proper terms, but I hope you get it)
Also I think that more people aren’t using Synfig for what it is created. It is focused on vector animation and should be more used in motion graphics stuff, and other work where vector graphics are used.