I used to ride in your position, but could never stretch out, this created some back pain. I always felt "scrunched", never able to effectively stretch out. Changed to a longer stem (80 to 100 mm) which helps a lot.

For you, neck pain is an obvious limitation. To gain more efficiency, you would have to get more aero, where the back is at least 45 degrees, or less upright. You appear to be closer to 60 to 70 degrees. Except this may increase neck pain. I wonder if a longer more upright stem or a longer stem with more spacers could help you get aero and allow you to keep your head up.

All the best...