The lower leg moving and pulling up is more a symptom of issues in the seat. When I find my extremities getting out of line, I go back to what is happening from the bottom of my ribcage through my upper thigh. If that is in good alignment and proper tone, the rest of the body will fall in line.
So instead of thinking about not tipping forward, I would think more about opening the hip angle, and getting the inner thigh flat against the saddle. Make sure your pelvis is more up towards the pommel so your seatbones are balanced in the middle of the saddle.
If your horse is reliable enough, I like to really stretch my hips out in free walk during warmup, with my stirrups dropped. Sit up stright, feel your seatbones in the saddle, and then alternately bring each leg out, get the inner thigh against the saddle so the toe is facing forward, and then bring the whole upper thigh back without tipping the torso forward. The motion of the walk will help loosen the muscles around the hip socket so your leg can drape. If my muscles start to cramp, I use the handle end of my whip to help loosen up the knots. You can always stop in the middle of your ride and repeat this if you start scrunching up again.
Also, if you can video yourself and review shortly after your ride, that can help a lot. People are having good luck with Pivo for this, and it’s much more affordable than the options that were out there in the past.