I think Sam and Bucky are going to end up working together. For one, Mackie and Stan's on screen chemistry is a license to print money, people love them so I doubt they're going to kill or retire either character any time soon. And despite their differences, they can't really say goodbye to one another. Their destinies are pretty much intertwined with everything they've both been through. The superhero community is really small in the MCU, there's no getting away from each other for very long.
I think they're going to have a symbiotic partnership by the end of the show. I think Sam is going to reject becoming a super soldier because he's seen what being turned in to a living weapon cost Steve, Buck, Natasha and now Isaiah Bradley. He doesn't want to lose his humanity, he wants to play football with his nephews and save his family's fishing boat. But without augmentation he's vulnerable, he'll need Bucky watching his back. And Bucky needs someone like Sam to help keep him anchored to his humanity now that Steve is gone, and helping vets come home again is what Sam was doing when Steve found him all the way back in CA:WS. And helping Sam is an avenue for Bucky to continue making amends and facing his ghosts.