Complex sentence: Even though he died at only age 26, he left an extensive body of work, including at least thirty-four symphonies and about thirty concertos, mainly for cello and for flute, though only about half have survived.
Split into simpler sentences: Even though he died at only age 26, he left an extensive body of work, including at least thirty-four symphonies. Although he composed about thirty concertos, mainly for cello and for flute, only about half have survived.