Complex sentence: She moved to the USA and remained with the Metropolitan for 20 seasons, singing some 30 roles, but she also performed frequently at the Royal Opera House, Covent Garden, Salzburg Festival, and the principal Italian houses.
Split into simpler sentences: She moved to the USA and remained with the Metropolitan for 20 seasons, singing some 30 roles. She also performed at the Royal Opera House, Covent Garden and the Salzburg Festival, as well as at the principal Italian houses.