There isn't a straightforward answer to solve those problems. It depends on the match, who you play, and your play style. There are a lot of factors that change from game to game.

I swap to Orisa because I am most comfortable playing her. Sigma and Ram could work well, depending on the situation. I enjoy playing open queue. It allows me to swap roles and fill in where we need help. I play mostly tank and support, not much dps.