I don’t think crunches are Satan (the whole spinal flexion myth etc). Not sure about crunches for powerlifting, 5/3/1 is not a powerlifting program.
While crunches are easier than other exercises (i.e. leg raises), I think if done correctly they can help with core stability.
Just use proper technique (it’s not difficult at all), hold the position at the top for a few seconds while exhaling, then slowly lower without dropping on the floor and keeping your back just a tad above the floor, then do the next rep. This keeps you rigid the whole time.
I’d take crunches over floor planks any day.
Btw, my ab/core work mostly consists of:
-leg raises, sometimes when my grip is fried I might do them on parallel bars, starting straight and going into an L-sit. They’re easier to do;
-loaded carries, namely farmer walks, suitcase carries and overhead carries. I like to do them at the end of the workout in some kind of circuit - i.e. doing 50 meters with a dumbbell in the right hand, then 50 meters with a dumbbell on the left hand, then 50 meters with a plate overhead, then repeat until I hit the amount of prescribed meters (250-400 depending on the template I’m running);
-if you want to sneak in some extra ab work, you could do some plank in your chinups. Keep the legs straight a bit in front of your body, squeeze your butt forward and keep your torso rigid, then pull yourself up. I’m forced to do it in my gym since the bar is a bit too low and definitely feel it in my abs;
as far as how to plan core work, assistance stuff is never set in stone, adapt it to your needs and see how/when it works best.
As a generic thing, when I was on an upper/lower template, four days a week each day a main lift, I usually did loaded carries twice (press and bench day), leg raises once (deadlift day) and back extensions or stiff leg deads once (squat day).
Now in the full body 3 days a week templates I’m doing leg raises once, loaded carries once and back extensions once - if the template has more than one deadlift day, I take off back extensions and replace them with either one between loaded carries or leg raises.