Though I thought Arthurian “legend” didn’t really settle until the early Medieval period, no..?

High Middle Ages (1000-1300), I believe.

Arthur himself supposedly lived in the 6th century, during the Anglo-Saxon invasions. There's also the anachronism that he defeats the 'Roman Emperor' long after there was no Roman Emperor except in Constantinople.

We really don’t know that much before Malory, do we, re Arthur..??

Actually, Malory was just a compiler. All of what he wrote about, which is not really knowledge as much as it is mythology, was just take from earlier sources and put it in a new form.

Unlike, say, the actual Macbeth…

Macbeth was an actual king, and supposedly a very good one - and the Duncan he killed was a young and incompetent ruler, not an old and saintly one.

Turns out it was King Charles after all. In England as well as France, monarchs named Charles were generally unsuccessful - with the notable exception of Charles VII.