Outside of losing Chadwick, the collision of Secret Invasion and The Marvels is/was the biggest reason for Marvel to pivot and slowdown
Feige wasn’t able to oversee both and both suffered from lack of vision and connectivity
Chadwick no doubt left a huge impact on the plans for Feige, but the problems they've had post Endgame were much deeper and were well beyond Boseman. Put it this way, post Endgame would still have the same issues even if Chadwick were still around.
And fwiw, I know this has been talked about at length, but the issues are less about lack of vision and connectivity and more because they never really recovered from all of the changes in their staff, directors, and overall drop in production value.
I'm not saying lack of vision and connectivity doesn't have anything to do with it, but there's a lot more to it than that. It's less connectivity and more just doing too much in too short a period of time. The need to slow down and bump up quality has been ongoing for a while now.