any suggestions on how to keep front hip closed longer?
Try thinking about driving your front heel to the catcher. I found it also helps to think about keeping your front side closed as long as you can, until foot strike. Work on it in your next bullpen, and challenge yourself to stay closed longer each throw until you are landing with your foot still closed. See how it feels, then find your timing on works best for you.
Also think rotating Hips DOWN into foot strike vs swinging open, ie hip opening at a downward angle similar to shoulder angle as opposed to straight across. Often the thought of creating hip/shoulder separation Can lead to 2 issues: trying to open them too much too early And opening straight across choking off energy transfer from your hips to shoulders which rotate at arm slot angle.
Yes, Tom House had a good drill called the get by me drill. have some one stand along your back side and in front a bit. use a chair or something else if no one avail. pitch the ball,