Regarding the setup, I love the space that they get.
Currently in the rabbit community, sand is very controversial and personally, I deem it unsafe as rabbits tend to ingest a lot of things that are in their environment as well as it is too fine for their noses.
As you have two rabbits, I recommend having a second litter box, especially after the intial bonding stage. Rabbits often get very territorial over their litter boxes and adding a second litter box that is away from the main one can give them the space to run away from the other if needed. The second one doesn't need to be as big, but since both of them are well over 5 pounds I'd say minimum 30" by 15". I'd also suggest a lot more hay in their litter box so they can dig in it as well.
Another thing I notice is that you have the tape/stickers and ink on the cardboard boxes still. It's still not 100% confirmed that Amazon uses a safe type of ink, so I would suggest to just rip the outer layer of cardboard off where there's ink, and also to remove the tape from the boxes to prevent blockages.
About their diet, as long as they're eating enough hay, it's fine for them to act hungry all the time because that's genuinely how rabbits are. 
4 cups of veggies a day is good, but I recommend more variety: think kale, parsely, mint, basil, dill, red leaf lettuce, spring mix, etc. Generally a rabbit should get at least 3 types of veggies a day.
I know you're on ROF as well so if you're looking for further advice I suggest you post there too! 
Hopefully this could help! 