I was also very impressed by this book, but not just because of the talent of the author (although Kazuo Ishiguro is certainly very talented). I'm not really comfortable explaining to other people why.
In any case, I will definitely read The Buried Giant.
As for Mary Stewart, I've read the trilogy and also "The Wicked Day" (where Mordred is a good guy and the main protagonist). When I was a little girl I also read a book by Rosemary Sutcliff about a Celtic boy who was born with a mutilated arm or something like that. I can't remember the title.
I can also name The Once and Future King and The Mists of Avalon. However, Morderd is the bad guy in these books.
There is also a book called "I am Mordred" by Nancy Springer.
Last year I tried to read a book that looked like a clumsy attempt to rewrite the Mary Stewart trilogy from Guenevere's point of view. I got through the first few chapters and gave up my unsuccessful attempts to read this book. The author's style was very good, but this book was like a dish made with very healthy and high quality foods, but lacking any seasoning.
I would hope that the books on your list are more tasty.