Is there a way to make all selected vertices of a path outline auto smooth?
Smoothing of vertices only apply on the go while using draw tool. So no.
As an alternative, you can use Inkscape to smooth your vector shapes and import them back into Synfig.
You are 100% on point, actually inkscape, does it better, because I use it I had just been over optimistic that synfig, has that tool too.