My opinion is that what a manager does in the clubhouse and how he handles the players has more of an impact on team results than most in-game decisions. As far as the in-game decisions go, I agree with you that managers don't move the needle very much.
Now, I'm not sure what is going on in the clubhouse, in particular, with regards to the whole Devers debacle dating back to when Bregman was first signed. Based on recent production, Devers seems to be okay with everything, but I can't help but wonder if his refusal to play 1B has created any rift in the clubhouse. As we know, when the team is playing well, things like this are a non-issue. When the the team is struggling, these things tend to be blown up more.
Either way, Cora may find himself in the hot seat if the team doesn't start playing better.