When I had a group of rabbits, they always had a medium coarse grass hay, refreshed twice a day, then I fed a salad of select leafy greens and forage in the evening. They were on a pellet free diet due to one rabbit having megacolon and being intolerant of sugars/high carb foods like pellets. The times that I did feed pellets, it was always just a minimal amount(less than the usual recommended amount), as a supplement to their hay.
If the fresh grass your rabbits eat, isn't a more mature long grass, maybe with some hard stem starting to grow, I would suggest adding a grass hay that is at least medium coarse, free fed. Reason being, in part what Blue Eyes mentioned about rabbits always needing access to hay or grass, but also soft early growth grass can be very rich and nutrient dense. It's not just grass and hay that helps a rabbits digestion, but the indigestible fiber in it. And it's the more mature growth and stalky stems that has the most indigestible fiber. Soft young early growth is more water, protein, nutrients, and digestible fiber, than indigestible fiber.
Indigestible fiber is what 'drives' the rabbits digestive system, and improves gastric motility. It increases the rabbits total output and increases the size of the fecal balls. In health terms, this keeps digestive content moving through the digestive tract effectively and helps balance digestive microflora. This is what helps prevent microflora imbalance and digestive illness from occurring, as well as helping prevent GI stasis developing due to digestive slowdown(provided the rabbit is still eating normally).
But you also can't just feed indigestible fiber and not enough nutrient rich food, or it can result in it's own set of digestive problems(eg. impacted cecum, fatty liver disease, weight loss). So it's finding the right balance of nutrient rich foods(eg. soft grass/leafy hay, pellets, veg, forage, etc) to provide the necessary nutrients for good health and good weight maintainence, with the right balance of coarse indigestible fiber for good gut motility, microflora balance, and gut health.
Instead of focusing on pellet amounts too much, I would focus primarily on the amount of grass/hay consumption. An adult pet rabbit should consume a pile of medium coarse grass/hay, at least the size of their body per day. If you have a softer grass/hay, then you'll want to find a way to balance this with adding a coarser hay as well, or feeding rabbit safe branches and leaves, and other safe forage with high amounts of indigestible fiber. As long as the hay and other forage, is a good balance of soft and coarse, the pellets are basically just an added treat and vitamin supplement to a rabbits diet, so not a lot needs to be fed usually.
https://rabbitwelfare.co.uk/wp-content/uploads/2013/05/article-ROWinter10p7.pdf
Because your rabbits are used to fresh grass every day, if they aren't used to hay they may be a bit picky about what hay they'll eat. You may need to experiment with different types and brands, to find one they like and will eat well. So it's best to buy a sample size to begin with, to make sure your rabbits like it and will eat it.
As with any diet changes, it's always important to monitor your rabbits weight and body condition, to make sure no unhealthy weight loss occurs.
