I don’t understand the idea that Novak has had any meaningful form advantage (whatever that really means). Nadal racked up half his career wins over Novak in a three-year period at the start of their respective careers. From June 2006 to May 2009, Nadal won 14 matches against Djokovic. In the next twelve years, he managed 14 more. Novak has no such period of such concentrated domination. That seems to indicate Nadal dominated a short span of time in which Novak was in relatively worse form, while Novak has been more reliably ahead in the rivalry since then – for the vast majority of their careers, through all the ups and downs of form.
Furthermore, they’ve played 32 matches in seasons where Nadal ended the year ranked higher, compared to 26 for Novak. They’ve met 30 times in years where Nadal won more slams than Novak, 17 times in years where Novak won more. This is with almost identical slam count overall. Their meetings have disproportionately occurred in seasons where Nadal was, by any reasonable metric, the better player, yet Djokovic leads overall. And this isn’t even getting into the surface skew!