This is one I'm working through as well, and I've just recently been able to do them on stuff above curb height. I don't switch ollie backwards, I switch ollie the direction I'm riding and slightly turn switch backside, so when I approach the ledge at an angle I can make it on top. I already rocket my switch ollies so landing with the back truck first has worked for me. This is one of those tricks that I don't think there's much of a specific technique to, I learned it by trying it over and over again, doing a bunch of switch backside willies. Learning this trick feels a lot like how it felt to learn normal backside 5050s, it's the same sort of thing where with your ollies becoming more confident and precise, your ability at the trick will improve. I agree with J soy that switch backside slappies are easier/ more comfortable, I learned those pretty quickly along with switch frontside slappies after I got normal frontside and backside slappies down and had thoroughly understood the mechanics of them.