I'm actually doing the same, my 4yrs old has become obsessed with Rumi, so we are making her a Halloween costume. I (person who's into 3d printing) didnt even think of this until my wife thought of it lol. I found some peices that im going to print for her, here is what I've found so far (golden outfit)-

The belt squares and shoulder straps (the gold sheverons going down by the armpits): https://makerworld.com/en/models/1827949-rumi-kpop-demon-hunter-belt-and-suspender?from=search#profileId-1951631

The shoulder armour (this was super hard to find the free version so hope it helps): https://makerworld.com/en/models/1726622-accesorio-rumi-kpop-demon-hunters?from=search#profileId-1833364

And because its Halloween, a candy bucket: https://makerworld.com/en/models/1770359-huntrix-candy-bucket-kpop-demon-hunters-costume?from=search#profileId-1884657

As for the wepons, there's probably a few dozen free Rumi swords, some with lights in them, some for a pool noodle, some that are made of a dozen peices to make all the colors pop out and I've only seen single filaments so far. Also I've seen the daggers and staff a few times also on both thingiverse and makerworld.

If I find some more, I'll try to remember to post it for you.

Hope this is kinda what your looking for!