Complex sentence: Music was a regular theme; he wrote most frequently about Wagner, but also about other favourite composers including Schubert and Mozart, and performers including Otto Klemperer, Alfred Brendel, and Kiri Te Kanawa.
Split into simpler sentences: Music was a regular theme; he wrote frequently about Wagner, and other favourite composers included Schubert and Mozart. He wrote about performers he admired, including Otto Klemperer, Alfred Brendel, and Kiri Te Kanawa.