I've used light weight spackle on foam and solid balsa wings. Its the one that feels empty when that container is picked up. Each one I've tried had slightly different characteristics, some remained soft, some didn't sand well, and some worked great. The current one I'm using is Lepage Polyfilla Big Hole Repair. It sticks and seem to sand well and there have been others I've liked but some I've hated too.
My experiences with red Bondo putty haven't been all that great either. When I could find it, the green 3M glazing putty was very good. Haven't seen it in years, tho.
Epoxy and microballons or the recommended additives should work. I use West Systems and their various additives. There's an optimum time frame between mostly cured and fully cured where the material can be be easily shaped as needed. Fairing filler is lighter and more easily shaped.
I have used spackle successfully for filling balsa grain but think it might chip if used to fill larger areas. I have used good ol' fill-n-finish to fill chips like that. Lots of them as I have used my broke down, dull bladed jigsaw for larger fins. I have never had a problem with the filler popping off or anything like that.