Complex sentence: She made only a few appearances internationally, choosing instead to work with important opera companies and orchestras in the United States; most notably the New York City Opera (NYCO) with whom she was active from 1958-1971.
Split into simpler sentences: She made only a few appearances internationally, choosing instead to work with important opera companies and orchestras in the United States. She was particularly active with the New York City Opera (NYCO) with whom she performed frequently from 1958-1971.