Wasn't Thordan the first Dragoon? He was wearing a grey or silver Drachen set in the flashback. I kinda think Estinien's blackened drachen armor is Thordan's that has just been blackened from battling dragonsn(from dragon breath scorching it, and dried blood which makes things black over time due to the iron in the blood which oxidizes). Dragonlancer is maybe a prototype which was lost/forgotten, while drachen was the more mass produced one. Drachen makes more sense from a functional standpoint however. The spines and spikes allow a dragoon, diving from the sky, to remain latched on to a flying dragon(the spikes pierce the scales) because if it was smooth armor, the dragoon would roll off the wyvern's back when it bucks. Also the dragonlancer gear appears more light and less armored, and his a superfluous tail thing which probably does nothing functionally.