The "hold to jump higher" it's actually pretty simple, you just need to multiply the Y velocity by a number that is less than 1 (i usually put 0.5) ONLY when the key for jump was released and the Y velocity is going up (you know, when the Y velocity is greather than 0 or less than 0, depending on the engine) i recommend you this video: https://www.youtube.com/watch?v=vFsJIrm2btU it explains a lot of tips to make your platformer controls feel a lot better.