I'm not an instructor or an experienced rider, but here is what I've found...FWIW:
First, don't have your weight too far forward. When your center of gravity is in the same place as the horse's center of gravity, you make it easier for the horse to balance. However, you also are at greater risk if the horse does something unexpected. Be at least a little bit behind the horse's center of gravity.
Second, depending on your style of riding, you can get deeper into the seat = more pressure on your butt. A loose leg helps with this since squeezing with the legs tends to raise your rump out of the deepest part of the saddle. I sometimes have to make a conscious effort to push my knees apart and to relax my legs. Weight flowing past your knees and into your heels lowers your center of gravity. If you squeeze with your thighs or, even worse, do what I tend to do and squeeze with your knees, you create a pivot point. In a sudden stop, you then tend to pivot around your knees and forward - face forward to the ground.
Third, it is OK to have your feet a little forward. Heels under hips works well with a well-trained horse moving with lots of collection. That doesn't describe my world. If your feet are a little forward and your weight flows into your heels, then in a sudden stop your feet help to brace you against the sudden stop - but even then, you want the weight flowing into your heels without any interruption. I'm not any kind of expert, but I like my heels in line with my belt buckle. Sometimes even slighly forward of that, since my mare Mia is fond of "The OMG Crouch"!
Fourth, I like my legs to be wrapped around the horse because that seems to help with sideways hops. Wrapped around doesn't mean squeezing, just wrapped. Draped. I started riding at 50. Given how tight my hips are, I have to have my toes stick out some to get my legs draped around my horse without tension in my legs or squeezing. I accept that because a relaxed leg letting the weight to flow into my heels is better than a stiff leg trying to hold my toes more forward.
Fifth, because my horse likes The OMG Crouch and hopping sideways to see if I'm still awake, I like a secure saddle. It isn't cheating to use tack that helps you. I'm fond of Australian stock saddles:
Notice the Mickey Mouse ears in the front. Those poleys help a lot when things go wrong. If I get too far forward, or my horse hits the fan, or she spins suddenly or stops without any notice, the poleys keep my hips in place and aligned with the horse's back. I also like safety stirrups. In western saddles, a wide pommel or swell can do some of the same thing, but I like the poleys because they are lower on my thighs and shaped to match the angle of my leg.
Again, I'm a nobody in riding. These are things that have helped me as I started learning.