You are correct the list is.
CR4 - Elephant
CR5 - Giant Crocodile, Giant Shark, Triceratops
CR6 - Mammoth
Don't forget about the elemental forms that unlock at level 10 those are pretty powerful CR5 and are size large.
As a house rule I could see a DM allowing the druid to add shambling mound CR6 to the list, older editions let the druid have plant forms. I might make it a reward for a sidequest, or if I don't have dinosaurs in my campaign world let him just have plant forms to make up for that diminishment to his abilities.