I think you start doing more HS using a wall. Get comfortable with that position, it will help your shoulder and core strength as well which will help your round-off. Either kick to a HS with your back against the wall or climb up the wall from a pushup position towards a HS. You don't need a trampoline for a round-off at all. You just need to develop a cartwheel that turns over very quickly and pops off your hands. Make sure you lunge enough, don't jump to your hands but place them on the ground one after another. In fact, you should pretend in a round-off that you are going to do a front handspring then change your hand placement at the last possible moment and use cartwheel hands.