how do i control my movement, so i can catch runners in boxshu/lenshu. ive practiced for hours trying to figure out a consistent strategy/launch technique, and every time i usually end up taking the entire game catching up to the person, or launch to the person, in a vulnerable way. does anyone have tips

You can't do much aside from the legendary crab technique. Always point your legs towards your opponent and launch like that. You have to play more if you can't do this. Keep practicing and you will learn it naturally

i always try to do the crab but it only works in lenshu, when i try it in boxshu, i usually just end up stuck in the wall at a weird angle, where extending my knee pushes me 3 inches, and me elbows dont bend so they i cant use them to move. my only chance is to swing my arms and legs and hope the momentum pushes me the right way

all i know is:
1.keep feet flat when u dont want to pivlot
2.put a let behind u
3.back falls are almost impossible to save so dont fall back

i figured out something that helped me, when you're falling down to crab position, relax an elbow or 2 so they bend a little bit, gives you more pop/ ability to change direction/launch angle

Yeah crab won't really help you in boxshu since boxshu operates with more frames. You will figure it out by yourself with enough practice.

learn the setups don't make ones up, and learn how these setups work and how you should launch from them.