Congratulations 500' is a major achievement!
There is variation in the drives which is normal after tweaking form. There are balance issues because the follow through step lands to different distances and sideways places. The grass hides how high the ball of the foot is in the air. You should see if the feeling is easier from the twisting point of view if you raise the ball of the foot more.
The elbow could move more forward still. The running speed and arm effort are fast grats on that. You seem to be in good shape.
You were semi rounding and do get far. How about testing straight pulls, Voigt style and double pivot as well with different reach backs?