A thought regarding your decline sit-ups:
From personal experience, I can tell you that I have felt the lower back strain you are probably referring to, and it was due to: 1. going up to or past a vertical torso position at the top of the rep; and 2. going down to touch my back to the bench at the bottom of the rep. I change directions at the bottom as soon as I feel my shirt graze the bench, and I change direction at the top just before I reach vertical - and I no longer have lower back pain in the decline sit-up.
Another thing - I hold the plate(s) behind my head, not on my chest, for 2 reasons: 1. you don’t have to use nearly as much weight; and 2. this position encourages proper upper back/scapular/neck posture, which will also decrease lower back dysfunction.
Hope this helps!