This is how the system works:
Game 0 you start at 2350 with 0 MMR confidence. Your SR is not visible until you finish placements. Your SR will move +/- 150-200 with each placement game depending on wins or losses. With each progressing game, your MMR gains more confidence, and the SR will move less and less with each game until it plateaus. This is why it is easier to move rank in the beginning, and harder after some time. Ultimately, how you move rank up is by winning.
For a deeper dive into it, you can read through here: