I haven’t been on his forum for more than a year.
But he generally likes to run 5’s pro and FSL 5x5 followed by assistance as 50-100 reps of push, pull, single leg or core work
50-100 reps push: pushup, db bench, dips (he loves dips), db press, flyes, any pressing movement.
50-100 reps pull: chin up, pull up (he loves those), bb row, db row, cable row any pulling movement.
50 - 100 reps Single leg: lunges, split squats, one leg dl, you get it
50-100 reps core: hanging leg raise (he likes them) ab wheel, planks, crunches.
The assistance should be light and non taxing.
All effort in the main lift, some in the supplemental, little to assistance.
Do 2 cycles of 5’s pro fsl one week deload, one cycle of PR sets and 3 - 5 sets FSL, and assistance is down to 25 reps.
one week deload one week test.
The test is to try to reach a 5 reps with the next cycles TM, that way you know your TM is correct.
I just happen to love the original way with pr sets I’ve tried the 5’s pro and pr cycle and test week. I didn’t like it. BUT a lot of peeps have had great success with it.
Sorry for the rant, I love 531 and think I know it quite well.
I think doing the opposite lift for supplemental would be good.
Assistance is really not that important.
You could focus on delts on the press day, quads on squat, you get it, or just do the push/pull/single leg or core thing.
I really would try to go with the template you put up, I like it a lot.
You’re working the opposite lift hard on supplemental and focus is on weakness on the main lift. Quality.