For my rabbits, I’ve found that keeping the diet fairly simple works best. The Rabbit.org Foundation has a really useful guide explaining how to properly feed rabbits and what proportion of their diet should come from hay, vegetables, pellets, and treats:
https://rabbit.org/care/food-diet/
What surprised me most was how important hay really is, because it should make up the vast majority of an adult rabbit’s diet. I now make sure fresh grass hay is available all the time rather than treating it as just another type of food. I also give fresh leafy greens every day and only a relatively small amount of good-quality pellets. Fruit and other sweet foods are something I personally keep as an occasional treat rather than part of the normal diet.
Since changing to that approach, feeding has become much easier because I’m no longer constantly wondering whether I should add something else. I’d still adjust the amounts or diet with a rabbit-savvy veterinarian if my rabbit developed health or weight problems.