What are some ways to develop a stronger lower leg (I've noticed my heel pops up a lot, and my leg swings forward over jumps)?

Bareback, for sure. I learned bareback before I ever sat in a saddle, and it helps immensely
Heel raises, seated leg press, squats, mountain climber... really any exercise you can feel in your legs.
No stirrups!
I used to have sucky legs and I started doing 200 squats a day🙄You prob don't need to do that many but that's what I did.

I also started off doing 5 minutes of no stirrup every.single.day. And then every day after that I would just try to get more than 5 minutes.

I can now ride for 30 minutes straight with no stirrups!

You really have to push yourself but it only took me like 3 months to be able to do 20 minutes so if you do this everyday it helps SOOOO much
I always use stairs. I will stand with the balls of my feet on the step, and then I lower my heels over the end of the step!
Off horse exercises -
Stand with the balls of your feet on a step and your heels over the edge. Slowly lower your heels down so you can feel the stretch in the back of your calves. Continue this exercise.
Complete 10-15 repetitions a couple times a day.
Also downward facing dog yoga positions help

I did this and it helped a lot!

Found this off google to help me with getting me lower leg stronger! 🙂
Try doing lots of no stirrup work while continuing to post and go in forward seat. Also work with sitting trot with stirrups, making sure your weight is in your heels.
If your leg is falling forward over jumps, then you are probably falling behind the horse. Try holding the mane so that you can get a better feel again for where you centre of gravity should be over jumps.
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