Pretty much what Aragorn said, but at the risk of giving you some information overload I have an additional suggestion.
Do your front squats as he described 1x a week.
On your second leg day, again start with front squats as Aragorn said, and then switch to back squats working up to one near max effort set of 5 reps. Then take about 50-60% of that 5 rep weight and do pause squats to the pins for 3 sets of 5.
Ideally you will set the pins just below parallel. For me, it usually works out that I bring them up one hole higher from where they were set for back squats as safety pins. Go down under control until the bar hits the pins, pause for about a second but DO NOT relax, and then explode up concentrating mainly on two things: 1) Lead with your chest 2) keep your knees out.
Ideally you should be able to work up to the point where you are doing 70-75% of your 5RM.
This will really help reenforce the correct movement pattern coming out of the hole. By pausing, the whole movement slows way down and you’ll really be able to concentrate on getting your chest up. As long as you keep your chest up, your back should stay flat. Not only will both the front and pause squats really help your leg strength, but I’ve also found they help with hip/groin/back flexibility as much as anything.
I’m also in the camp that you should not deadlift at all for a while. Ideally, you would completely forgot how to deadlift and rewrite that motor pattern in a few months after your squat intensive cycle. If you feel like you must deadlift, I’d suggest giving sumo a shot for a while.
As for your assistance work I really don’t think it matters much. Go for a GHR or reverse hyper if you have access, otherwise just make sure you are squatting with enough intensity that you don’t even need it. Personally, split squats and lunges are favorites of mine, but I definitely wouldn’t recommend you do the split squats. It’s just too easy to bend forward at the hips and you’ll end up rounding your back again. Lunges may be worth a shot. I’d cut out anything and everything that causes you to round your back, though, including good mornings. Again, we need your body to forget that movement and that won’t happen if you keep doing exercises that reenforce it.