I've mixed up epoxy with wood flour to a peanut-butter thickness, but it has a pretty rough texture. The denatured alcohol trick from the manual helps, but it still needs sanding. Is there a better filler or combination of fillers that results in a smooth, strong and relatively sandable fillet?

It helps if you add some silica powder to your woodflour "peanut butter" mix.   The mix should be about 10% silica powder and 90% wood flour. That really smooths out fillets.