Sit ups? Alternative to using bench?

Hi all,

I hope this is the right forum. I have lower back issues on and off, so much so that doing sit ups was always a chore to me.
I had an angled sit up bench that worked, but I could still feel the lower back sometimes. I've been at the physio recently, and sold on the sit up bench now, as my back is just too dodgy.

I've been shown some good core exercises I can do to strengthen my lower back muscles.

Anything else I could do to focus on the stomach muscles? Planking maybe?

Or any other apparatus out there I could use sitting down or standing up for example?

I'm not looking for washboard abs, just exercises to do along with everything else...