It's basically this although I do both legs and both arms at the same time
Doesn't look like much but if you're doing the reps slowly and really concentrating on keeping your posterior pelvic tilt it catches up to you!
My back's actually been really, really good lately. All I do for lower body these days pretty much is front squat and good mornings. The good mornings really have been a godsend. I actually think my horrendous mobility might be improving a little.
Definitely learn to brace. I brace like fuck when I do pretty much anything that's going to involve my lower back. Even little things like picking a heavy dumbell up I'll brace for.
I don't back squat or deadlift, and don't think I ever will again. Between the front squats and high rep (15-20) good mornings, I've pretty much found a way I can load my lower body up and train it heavy (the FS, not the GM) so I see no reason to take the chance and try to back squat or deadlift again. I do occasionally glance over at the trap bar like mayyyybe but I always decide against it.
Humour me - when your back heals up switch to front squatting and high rep good mornings (making sure you really brace even on the "light" good mornings). If it worked for me it might work for you.
As for the SI joint thing, a good piriformis stretch can usually provide pretty much instant relief when it tightens up.