The difference in reliability of both the car and Fourmaux is indeed impressive.

For the car I wonder if this is due to less development/push for changes, compared to what seemed to be there with Tanak.

For Fourmaux he is much better at driving at little bit less than maximum push and keeping it on the road. That said pacewise he mostly goes around 5th place in similar conditions. Whether that is due to the car not being as fast as others or due to him backing is impossible to tell. Therefore it's kinda early to call him a finished article, especially after the big error in Croatia.