Sometimes when I’m getting too steep with the driver back swing, or when I’m deliberately demonstrating or practicing a "one plane" swing, I try to keep my back elbow closer to my side by letting it fold relatively earlier in the back swing.....more and longer "connection". By keeping the back elbow "in" it will tend to stay "down", thus forcing a flatter back swing. This may not be exactly what IS happening, but this is what I try to FEEL is happening. Just something you might try.