Hmm, I were right under 3500 when I quit ( probably gonna pick up ow again soon hmm )
Well, what I did was that I only played Mccree & Soldier and nothing else. Ended up getting really good at those, I always had a positive winrate so overall I gained rank.
Eventually I went from silver to 3400 this way.
Before that playing flex from season 1 - like season 6 I never really gained any rank, stuck in silver - gold but when I picked 2 mains I gained SR quick