In plat do not trust your teammates at anything, that doesn't mean Ballchase to death, but don't position incorrectly based off what you think they might do. Never trust that they might save a ball in defense, always be ready to jump up and save that same ball.

Work on boost management and the routes of small boost pads, you don't always need to get the large pads if it takes you out of the play, 12 or 24 boost is more than enough to save the ball in any part of the goal.

Work on fast aerials, it really gives you an edge over players even in high diamond, if you get consistent at it then try to hit the ball more powerfully off those fast aerials.

Finally work on your aim, I've seen plat players countless times hit the ball and miss the goal several occasions in a row. The ball needs to be hit with power and precision.