You could probably run more cycles before resetting on 531, but just to throw this out there:
It's not uncommon to have to reset your training maxes and if you had to do it on 5x5 you will have to do it again on 531. I don't really think it matters what program you follow as long as you aim for progression (more reps or more weight or both) and train your weaknesses. The latter is probably what you need to think about more with respect to your assistance work.
Finally, have you gained weight on the program? Are your figure competition goals keeping you from eating enough to progress on a powerlifting program? You might want to increase calories and take your bodyweight up 5-10 pounds, or give it the freedom to do so, in order to continue progressing.