No, I am not an athletic kind.
The L5 issue is gradually improving. I suspect I had the disc/vertebra issue since childhood as I had episodes of lower back pain on and off for 30 years. I think both lower back and neck issues are connected to the poor posture, but probably are independent from each other. I tried different back exercises, but none had any obvious benefit. I learned by trial and error which exercises trigger the foot numbness and avoided doing these for a while. I am now able to do some of these again.

I am a bit concerned about abdominal crunches too. I make sure I always complement these with upper back strengthening to counteract any effect they may have on the posture. I am also doing various leg slides like you suggested (and also with one leg raised) though I find them too light of an exercise.