I've read nearly a dozen 'historical' books on the possible existence of a post or sub- Roman Brithonic leader named Arthur. I put 'historical' in quotes because, of course, there is no contemporary written account of such a person. The first use of that name was in the Historia Brittorum written in 828, which was more than 300 years after Arthur's time.

The only book I've read which offers a logical explanation for a real Arthur who was located in a specific place and approximate time is Flint Johnson's "The British Heroic Age" (2017). OCLC- 948561022

It is unique because it makes the case that the original written source for an Arthur was a so-called "Northern memorandum" which no longer exists. Johnson claims that the memorandum was begun in 590-630 by Ruhn son of Urien, a famous chieftain in Rheged, a kingdom in what is now northwest England. Johnson argues that the account included mention of a local battle leader named Arthur in the Carlisle area, and that the account survived as the memorandum was used for political purposes first by Urien's family, then by the kings of Northumbria, and finally by the king of Powys in Northern Wales. It was there that the document's Arthur story was incorporated into the Historia Brittonum by it's Welsh author, believed to be a monk named Nennius.

Nennius was the one who took that Arthur and made him the hero of the Battle of Badon. And from there future story tellers added all the medieval trappings of Camelot and the round table.