Complex sentence: As concertmaster, he assumed the principal responsibility for composing new works, specifically cantatas for the "Schlosskirche" (palace church), on a monthly schedule, aiming at a complete annual cycle within four years.
Split into simpler sentences: As concertmaster, he assumed principal responsibility for composing new works. Specifically was tasked with providing cantatas for the "Schlosskirche" (palace church) on a monthly schedule, and a complete annual cycle within four years.