DH has diverticulitis as well. He almost died from his first bout and has since had his lower intestine removed.

the main things are:
Keep hydrated
Have fibre
Eat clean
Dont get constipated.
I make DH a bircher muesli for breakfast most days, he finds if he does not have it he feels awful. It is made with yoghurt, plus he has extra yoghurt when he serves it up.
He avoids breads with seeds in them, even though he does not avoid tomato seeds (maybe because he loves tomatoes)

He takes lactulose if he is feeling constipated.