Barry,
Richard Strauss was not Wagner. Wagner wrote the librettos to all his mature operas, and he wrote voluminously about opera and music and his own works. Works like "The Art Work of the Future" and "Opera and Drama" and many others plus an extensive autobiography--so while many of us may not be particularly interested in philosophy and music, Wagner was. His study of Kant and Schopenhauer, as Magee demonstrates, was intensive and extensive through his mature life. His discovery of Schopenhauer in 1854 certainly developed quickly into his writing of "Tristan" and deeply influenced his reimagining of the Ring, the meditative aspects of Meistersinger, and perhaps the whole of Parsifal.
Wagner also had extensive discussions and arguments about Kant and Schopenhauer with one of the most powerful (and dangerous) philosophers of the modern world, Nietzsche. Nietzsche wrote his first major work, "The Birth of Tragedy" under the spell of Wagner and "Tristan." Of course Nietzsche felt it necessary to take the strongest possible opposition to Wagner with Parsifal in his late works--so much afoot there, So while these things might not appeal to many of us, Wagner's deep connection to philosophy is undeniable.
All the best, Barry, and thanks for your many posts to this list. I've learned a great deal from them and others over the last few months.
Tom (in Vermont)