There are writers who do Sam/Steve. I think harmonyangel is one. I don't know if her fic is flocked, but she has an AO3 account. gloss writes a lot of Sam Wilson fic. Not necessarily Sam/Steve (although there is that), but her stuff is well written, no matter which way you slice it. I'd say, if you can't get a hold of 616 Sam, I'd say 1610 Ultimates Sam is similar enough (intense and very proud) - but Ults Sam is a scientist and a former member of the armed reserves, whereas 616 Sam was a pimp (Marvel was in a bad place in the 1970s) before he became a social worker.

I think in order to write Steve and Sam versus Steve and Tony, you have to realise that Steve likes each guy for his own merits, and Steve isn't one to compare the two. I've noticed that anyway - if either guy (Sam or Tony) tries to object to the other's presence in Steve's life, Steve just doesn't... go there.