Complex sentence: John Bacon, a talented sculptor, had worked for Mrs Coade since 1769 so in 1771 she appointed him as works supervisor, he then took over both model-making and design until his death in 1799.
Split into simpler sentences: John Bacon, a talented sculptor, had worked for Mrs Coade since 1769. In 1771 she appointed him as works supervisor; he directed both model-making and design until his death in 1799.