>>715008490Don't grind the same section of rail multiple times in a row, and don't manual too many times in a row, either. Those give you big penalties to your balance. When you're on a rail, you should be pressing the normal inputs to switch up your grind, not ollieing off, then landing in a different grind.
You can buffer the input for a revert to make it more consistent, too.
50 Million points isn't actually all that much for a single combo, it's very achievable even if you're not that skilled, as long as you have a plan.
In general, start out with your highest scoring special tricks. Frontload all of them to the best of your ability, then spend the rest of the combo chaining together low point value tricks to build up your multiplier.