Braking is a fine art, no doubt. It’s good to practice using more front brake, as that provides most stopping power, and it’s a good skill to have in emergencies. Front brake is safest when you’re travelling in a straight line and have your body weight evenly balanced over the bike - you definitely need to be careful using front brake while cornering, or riding over icy/slippy ground (any situation where there is a lateral load, your weight is off centre, or there is so little grip that the front wheel can slip out sideways). But it is important to learn how to use the front brake as much as possible, while avoiding dangerous situations. Apologies if you know all this already!
Yep - downhill at speed in low grip conditions (eg gravel) can be scary. Once your rear wheel is locking and you still need to slow, then front wheel braking becomes essential - and then it is a question of how much you can apply before pain becomes the inevitable result