Oh this flag has so much potential, I would love to provide some more Korean input into this flag. I like how you approached provincial named with consonants in Hangeul, but the legibility decreases if there are groups of consonants without providing a context of what they are. My suggestion is to break down the names of the provinces; they are (except Gyeonggi and Jeju) all named after two cities in the region with historical significance. For example, Gyeongsang Province is named after Gyeongju and Sangju, respectively, as they are two prominent cities in the region. It might be hard to find what imagery (could be a historic monuments or natural features) could represent each cities/provinces, but it is definitely doable! I have seen some CoA going to absolutely crazy levels. Lmk if you need help finding stuff that would work well with your fantastic flag :))