Spackle works fine, it's the best filler over raw foam because it sands at about the same rate as foam or balsa. Adds very little weight.
I've never tried balsa sawdust with epoxy.
Balsa sawdust and CA sand at the same rate as concrete.
If spackle is encapsulated with epoxy, paint, BalsaRite, etc., it holds up OK under cloth or film. It is a ding filler, not for structural repairs.