Complex sentence: He was instead given roles such as washing the windows but he found his creative niche in arranging the store's front display that was so impressive that his boss, after his first attempt, assigned him that role from thereafter.
Split into simpler sentences: He was instead given jobs such as washing the windows where he found a creative niche in arranging the store's front display. His work was so impressive that his boss, after his first attempt, assigned Woolworth that role from thereafter.