but the down side of those, if they're the type that my wife bought, is
that
they don't sand well...

Point of fact, Solid glass micro-spheres are a b*tch to sand. However,
hollow glass micro-balloons sand extremely well. They are the resin filler
in the 'light' autobody fillers.

Other good epoxy fillers are talc (easy to sand, filler used in most
autobody fillers), phenolic micro-balloons (purplish-brown in color),
mini-fibers (white polyester), Cab-O-Sil (fumed silica thickener). I've
also used ground walnut shells, and calcium carbonate (ground marble).