Watched Endgame again yesterday. And then watched the Director's Round Table afterwards. And really looking back at it, I don't know if it was directly the Russo's or Marvel/Disney willing to work with other directors that lead to the huge change in the films.
But look back at early films and the later films and you can basically see when things took off.
Iron Man (Favreau)
Hulk (letterier)
Iron Man 2 (Favreau)
Thor (Branagh)
Captain America (Johnston)
Avengers (Whedon)
Iron Man 3 (Black)
Now here's the break. Look at that list of directors, it's mostly underwhelming and the non-big names are mainly action movie guys. Which is why I think they had kind of a Transformers like feel (Iron Man 3 for example)
Winter Soldier (Russo bros)
Guardians (gunn)
Ultron (Whedon)
Antman (Reed)
Civil War (Russo Bros)
Doctor Strange (Derrickson)
Guardians 2 (Gunn)
Spideman (Watts)
Ragnorok (Waititi)
Black Panther (Coogler)
Now compare the 2, while still many not well knowns were brought in their backgrounds were starkly different than the majority of early directors. Russos were known for their work on Comedies, shows like Arrested Development, Community, and You Me and Dupree. They obviously had skills at action set pieces but they brought the comedy and heart from sitcoms to the MCU. Gunn was into black comedy and horror directing films like Super and Slither and writing films like Scooby Doo and the Dawn of the Dead remake. Reed spent a lot of time in Comedy but more specifically Rom-Coms directing films likeBring it on and The Break Up. Derrickson a lot of time in Horror and Sci-Fi Exorcism of Emily Rose, Sinister, the Day the Earth Stood Still. Watts worked in horror and action and comedy before hand but his catalogue is a bit more bare than some of the others. Taika was all over the place, doing romcoms, dramedies, and a horror mockumentary. Coogler had done 2 films before Black Pander, Fruitvale Station and Creed. So he brought some heavy drama chops.
And again, I don't know if this was a conscious choice by Marvel/Disney to try somthing else or if after the success of Winter Soldier and GotG they decided to specifically avoid action directors and go with more drama/comedy guys and gals. And this continued too. Boden and Fleck, huge drama chops, Shortland, Drama and horror, Cretton big drama background, Zhao huge drama chops, Raimi huge horror chops, he of course did other things (spiderman and oz) , but horror is what he did the most, and DaCosta, Horror and drama.