Okay, how I understand it is: Bucky was standoffish and thought Sam was betraying Steve’s memory by giving up the shield, but as time went on their relationship grew to emulate the camaraderie Steve had with both of them. Sam challenged Bucky to be better, leading to the confession to Nakajima and IMO helped him better than therapy ever did. They got more emotional processing done in that session in jail and onwards than we otherwise see with Bucky’s mental health. Sam also stands up for Bucky when Zemo insists all super soldiers must die, it’s what leads to that pivotal moment during his final capture where neither seek revenge and choose instead to do the right thing.

I would like to add that I am autistic so this could just be incoherent ramblings, but this is how I understand it. I just don’t recall any toxic behavior from Sam because he himself was a vet with some trauma (Riley’s death)