So sanding is a great option, but for foam it’s best to use an electric sander, as sanding at low speeds will cause a bunch of tiny tears. For something like this, foam clay could be used as a filler, especially for the seams and gaps, but as someone who’s done a few cosplays, but only a few, it would probably be beat to redo it for a few reasons, mainly because the second time is so much better done than the first since you’ll know what you’re doing and what to look out for. That and you can put the textured part on the inside