Solo MMR is mostly about luck..
my fren was once 5k, now dropped to 4.7k ish.. and he gave up on it. lol

On topic, i think a strong mid will win half of the game.
By strong mid, it can be tempo controller, or a semi hard carry.
Pick base on situation, if ur team have more than 2 carry, go tempo controller like a quas wex hunting invoker, qop, etc. Even slark mid with ganking item will do well.

If ur team have 1 carry, and u have no faith in that carry. try picking semi hard carry, example SF on radiant ( I love dis ), stack nc while at mid. Very easy farmed up. Or even the new meta sniper mid, it's good.

For tempo controller, gank more. Don't keep farm, create space for ur carries, makes enemy focus on u.
For semi-hard carry, only participate in war that u think ur team can win or have an edge, don't force fight too often. Get your core items up.