I had similar history with my back. It wasn't a laundry basket but...
I think your should sprinkle more core work in amongst your workouts.
Planks are definitely great. Besides front and side planks you should still do some side flexion sometimes, and some rotation such as russian twists or cable woodchops. Some anti-rotations such as pallof-press or anit-rotation barbell (Pity's favorite). And I'm not sure if you already do some flexion but reverse curls are good. I really like Janda situps for a very intense ab workout. But I think the static moves are the most important and the anti-rotation. Anything that is about stability.
The big thing to be looking at, amongst all the other big things, lol, is where the movement is coming from. If you are getting hypermobility in the lower back and not enough mobility in the hips/ankles...you need to change that. Reverse it.
I'd be interested in how you do those pullthroughs, btw. Many do the lumbar flexion/extension style. I take it you are not doing this and it is a hip dominant (glute/ham) move.
Let me guess..those back stretches feel good. When you have a bad lower back, lower back stretches are like picking a scab...feels great but it's bad for you in the long run.
There is tons of info here:
http://www.sciatica.org/