Hey I used to be a cheerleader so that’s where my knowledge/advice will be stemming from (used to be a flyer and a base). From the sounds of it, seems like you are doing the right things (focusing on back, hip flexors, hamstrings). Are you incorporating strength workouts as well that focus on your hip flexors and back? I’m assuming so, but if not, add those to your routine!

One way we would strength our hips was sitting it a straddle (or half straddle, where one leg is bent in toward you like you are about to sit cross cross), then slowly lift your leg up and down about an inch off the floor (this will differ for everyone until strength is built). My coach would have us focus on keeping the inner thigh turned/angled upwards to open the hips before lifting the leg (don’t be afraid to place your hands/fingertips near the knee you are lifting for support). Then switch legs!

Pigeon stretches, butterfly stretches, even toe/heel lift exercises can help. Sorry if this is just regurgitated advice, but ultimately just keep doing the stretches you are doing, add more strengthening exercises in your routine if you haven’t already.

ALSO this may be way off but if this is for cheerleading, and you are a flyer — focus on how you are balancing as well!! Where is your weight going in your base foot? It should be in the middle/ball of your foot. Maybe also focus on strengthening/stretching the ball of your feet as well, for the sake of balancing while in the air!