WELCOME Skeeternosky! As long as she is munching on her hay and eating veggies, then it is probably due to the pellets only. But if she’s not eating anything, then you need to seek out a vet.
If it is the pellets. Do you have any of the old pellets to test if she will eat those? Anytime you switch pellets, you should do so gradually (over 3 weeks) to prevent digestive issues. Each week weeding out the old pellet a bit more.
Nibbling on cardboard is normal, but rabbits that resort to actually consuming their litter pellets and cardboard may need more fiber. Does she eat alot of her hay? Many times rabbits who start eating more paper and cardboard can sometimes mean they need more fiber in their diet. How much fiber does your new pellet have?
Also when you said you switched to a brand that has more variety – what does that actually mean. It makes me think of the ones that have seeds, and other things in it along with the pellet, and most of the time, those are lower in fiber, higher in fat and sugars, Not the best.
Look at the label post the contents and nutritional value.
Also, just as a reminder, if she isn’t eating AT ALL, and/or not pooping or drinking you will need to call a vet
UPDATE: I just read your second post. So she stopped eating her first pellets. How are her teeth? Have you noticed any drooling, or wet fur around her eyes?