Lockout is all about GLUTES - whether it is making them stronger, or tinkering with technique so that your hips are in the optimal position/angle for them to be used most effectively. Any exercise that targets the glutes will help you.
Other than adding glute strength, the higher your hips are at the start, the more hamstring you can engage for speed/power off of the floor. If your hips are too high, however, your legs are locked out or almost locked out with still too much distance for the bar to travel and the glutes are at a disadvantage. I would say have someone watch or video your sets the next couple of weeks and tinker with how high your hips start to find that sweet spot between speed off of the floor and solid lockout.
Once you have established that, start pulling from a deficit for 3 sets of 10 once or twice a week. Think of it as an assistance exercise, so not all that heavy–my competition best is 375, but I only use between 225 and 245 for the deficit reps (I’m a girl in the 148 weight class, by the way–please don’t take advice from a male that pulls less than 500, lol). A former guy I used to lift with uses between 315 and 335 and is planning to hit 600 this weekend in competition in the 198 weight class.
Deficits also train hamstrings and floor speed a ton, but the longer range of motion will help you feel when the movement switches from hamstring to glute, and the volume will give the the ability to grind through a rep that you might struggle to lock out. I also like what someone else a little earlier on this blog said about adding in goodmornings.
Rack pulls are partial, and unless you can manage to do them in the exact same angle you end up in when pulling from the floor, I don’t know that they are worth it .