This all sounds really good! You’ve put some great research into this, and I’m sure you’re going to be an incredible rabbit owner! I have a 5 month old Holland Lop myself, and I know you’re going to love yours. Have you given any thoughts to names yet?
I do have to agree that even if your rabbit is alone more, I think it will be better to have just one pen, at least at first. When she(?) gets home, she’ll have a lot to adjust to, and I think she will feel more at home if she has one permanent spot than if she’s moving between two. It also makes litter training so much easier. The great things about rabbits are that they’re crepuscular and generally sleep for most of the afternoon, so I wouldn’t worry too much about the hours that she’s by herself. Between that and the time she spends outside her pen, I think she’ll be okay. Or are you just planning on using one of the pens as a play area? If you introduce that later after she’s already comfortable in her space and it’s clear to her that the other pen is where she spends most of her time, that may work.
I’m also curious whether there’s a way for you get the rabbit from your room to the area where her cage is without carrying her there (which may not be what you’re planning to do at all!). In my (albeit limited) experience, baby rabbits are sometimes more receptive to being picked up before they reach puberty, but even so, most rabbits really don’t enjoy being picked up. Just something to think about so that you can help her trust you (and again, I have no idea what your plans are, so ignore this if you have a different strategy!).
If the jingling balls are hard plastic, I think they’ll work as long as your rabbit isn’t able make a dent in them. Is the cardboard box a sort of a hidey home that your rabbit can go into? If so, your supplies sound perfect.
As far as changing pellets go, rabbits should be on their adult diets by about seven months. I’m planning to start my switch within the next few weeks, as I believe that smaller rabbits stop growing sooner. I don’t think it would hurt for you to put off the switch slightly later or to introduce vegetables slightly soon. Also, don’t be concerned about introducing vegetables at the same time as unlimited pellets. Honestly, though, I don’t think there will be any negative ramifications either way. As long as you’re watching what your rabbit’s eating/how she’s reacting, which it sounds like you’re doing, she should be fine! And don’t feel bad if you have to limit your rabbit’s pellets a little bit if she’s not eating hay as much as she should, since hay is the most important part of the diet.
I think my biggest advice is just to remain open and to enjoy the adventure of rabbit raising! Like you, I’m a planner, and I wanted to have everything perfect by the time my rabbit came home. Unfortunately, I discovered I couldn’t do that. My rabbit turned out to be more of a climber and explorer than a player and snubbed some of the toys I got him, and he hated the first brush I used on him. Since it seems like you’ve already gotten all of the necessities, I would hold off from buying anything else until you know your rabbit better (which you might not need to do for a while…yay for DIY!).