I picked up Lose Your Mummy Tummy by Julie Tupler to help with my self-diagnosed Diastasis. I'm still reading it, but so far, it seems like a go-to exercise is sucking in my gut. The author talks about imagining your abs as an elevator (from front to back) and getting your elevator to level six--which sounds like maybe your belly button is pressed against your back. These elevator exercises are supposed to strengthen the transverse abs, which will hold together the obliques and recti.
There are a couple short workouts in the book, but so far, I haven't read them. And I'm not sure how to fit more workouts into my day. So I'm doing Kegels (which there is debate about the efficacy of) and sucking my gut in.
I've set the book next to my "Nursing Station" so I can pick it up when I sit with Livy.