Sorry to be posting so long after your posts, but I'm new to this interchange. For what it's worth, and maybe not a whole lot, here are some comments about thumb rings and the Kaya (correct pronunciation Gaya as the Korean spelling is 가야 and the ㄱ letter is pronounced as a "g". It's only a "k" when at the end of a word: 국궁, Gook Goong, the name for traditional Korean archery.) I hope the Korean words I've written haven't come out as gibberish.
I looked at an American internet retailer who sells the Gaya bow and thumb rings and am commenting on what is available there. First the thumb ring. There are two types of Korean thumb rings. The one being sold by this retailer is the ahm gawk chi. It's the one that is a blade that covers the last pad of the thumb. In first using it, it feels quite restrictive as there isn't a lot of room between arrow, string, thumb ring/thumb and index finger. With the arrow nocked, the thumb ring and thumb are placed below the arrow's nock. The ring should not be in contact with the bottom of the nock, but rather the upper edge of the ring should be about five to six millimetres below the nock. One would think that the ring would slide up the string on drawing, but the ring is anchored in place by pressing the side of the index finger at the first joint/knuckle against the nock. This happens when the index finger is brought forward and wrapped down, around the thumb's nail. A very slight lift or rotation of the hand upward adds a bit more pressure against the nock. This does the mentioned anchoring and it also holds the arrow in the "V" rest formed by the side of the limb and the gripping hand's thumb, even with the bow slightly canted. All this is a rather tight fit.
In looking at the string provided with the Gaya Bow, it is one of two styles of "carbon-bow" strings used with this bow. (There is also the horn-bow style string that can be used.) The difference in the two styles is the serving on the loops. The one pictured is the one where there is serving on the contact points with the nock and the nock pads. The other has the loops completely served. The latter lasts a lot longer, but is more temperamental when stringing the bow.
Korean archers use a second serving on the centre serving. They use a length of string material, often taken from an old string and a part that hasn't frayed or gone fuzzy. Knowing where this is placed, consider this: Koreans nock the arrow about 1 1/2 diameters of the arrow above the point where the arrow would be at a right-angle (90 degrees) to string. Knowing the nocking point, the second serving starts about 5 mm above it and finishes maybe a centimetre below. Why the second serving? It eliminates wear to the main serving, but maybe more importantly, it focuses the archer closer to where the arrow is nocked.
My apologies for being so long winded, or confusing in the detail given. Take it all for what it is. It might be useful or then again, irrelevant. Anyway, enjoy the Korean bow for all it's worth. I personally don't like the Gaya bow, but my wife does; she has three. All the best and many hits.