@dawn16 I've always been pretty naturally gifted when it comes to squats so I haven't spent as much time strategizing for them, but I would say the same principles apply: breaking the movement down into it component parts, being present throughout the movement, identifying the limiting factors, and targeting them.
Pistol squats in particular are often limited more by flexibility, balance, and isometric stability than they are strength. The muscle groups involved are some of the largest and most regularly activated in the body and in general shouldn't be too far off from the power capacity to move one's body weight.
So, component supporting exercises I can think of off the dome to help would be:
-Glute bridges/hip thrusts w/ isometric hold. Single leg and/or weighted variants if you can manage. I notice a lot of times the part people really struggle with pistol squats isnt so much the leg extension/locking out, but rather the hip/glute strength and getting out of that lowest part of the squat steadily and safely.
-Good mornings/russian deadlift if you have access to weight. Can be done with a db or kettlebell, or really by holding anything heavy. Single leg varient in particular I think would help with pistol squat. (This is for core stability and lower back support so you're not getting that bend that pistol squats sometimes encourage)
-Lunges, weighted if possible. (For obvious reasons)
-Sumo squats (for hip flexibility and range of motion).
-Standing or hanging leg raises (for keeping that leg straight and stable so you can focus on the squat itself.)
And then lots of stretching so you're not fighting yourself so much. Hamstring stretches, hip flexor stretches, leg raises, lateral hip stretches, etc. I would also recommend some balance-oriented yoga poses or just practicing standing on one foot if that's a big holdup for you.
Another thing you can do to help get the ROM down is to use a prop to set your off-heel on to start to get your body used to maintaining that form without having to fully balance. You can start with a short prop and steadily raise it as your mobility and balance improve.
