I know wood filler is quite brittle, and takes very long to dry if applied thick, also is kinda expensive for bulk use. I haven't worked with spackling specifically before, but I think it's just a pre-mixed drywall compound isn't it? If you were forced to choose only between these two I'd prolly go with that, I know people manage to make relief sculptures with it and stuff, so I imagine it's a good bit stronger. I really question how well either will handle any sort of flexing though that the cardboard will inevitably give, I can see them both crumbling off very easily. Usually I recommend stuff like apoxie sculpt for people looking for non-toxic alternatives to bondo, but I'm not sure how well it would adhere to cardboard. Maybe a paste-like paper mache mix would work well?
I've used Mod Podge as a EVA foam sealant before, it's nice because it's sandable. It's pretty much just elmers glue with some sorta extra hardening compound in it that lets it dry super solid, so it'll work great with cardboard. I haven't used polycrylic before, but I assume it's just a non-toxic alternative to polyurethene, which will work too, but it'll probably be really runny. Wood finishes are generally very thin and intended for flat horizontal application.
Internal filling is very usefully for providing stability and rigidity, if this is gonna be a wear helmet though it can take up valuable internal space for your head and internal accessories. It's sorta a careful balance.
Whatever different materials you choose though, try to use all of the same one or at least ones that have similar sandability qualities. The difference between lumpy and beautifully smooth helms is 100% the amount of sanding that goes into it, it probably takes up 90% of the time that goes into a good build. Having materials that sand differently or gum up papers quickly can make that experience a nightmare.
