One of my buns has major jaw issues and basically only has teeth on one side (top & bottom), top incisors and one lower incisor - he is managing to maintain his weight OK with minimal intervention. Dandelions and softer leaves are OK - herbs (mint, coriander, etc) celery leaves, rocket; also broccoli florets & similar. Sometimes he needs food softening (pellets), chopping (cabbage) or grating (carrot, apple), but he prefers to forage.
He struggles with coarse grass, so soft bits only - and he can't eat hay any more, although he does try.