King Arthur, the Making of the Legend

Rheothemus maybe is the strongest candidate we can point to. But he contradicts dates found in annales cambrier which put arthur's career in the early five hundreds, not back in the four hundreds. And thirdly, as for rheothamus, this leader of britons who landed in gaul and tried to fight together in alliance with the romans. So all of these candidates are just very weak and too far afield to connect with any confidence to the historical king arthur. Could there maybe be some piece of inspiration, or some scrap of raw material here that fed into the legends around king arthur?