The best advice I can give you about the Shuttle is get to the line early. The trip only takes about 7 minutes, but if you wait until later in the day, the lane could be several shuttles long. There are plenty of horror stories about missing shows and what not because of shuttle lines. As for what you could bring, I’ve always had my backpack with everything I needed for the day and my chair with me. Be prepared to have to store things on your lap if you are sitting, however.
Best advice I can give is that you ride it only twice per day - once into town, and once back to camp at the end of the day. Pack everything in the morning, and take to the fest everything you’ll need to survive the day, the heat, the night, the cold, the possibly wet, the hunger, the thirst and everything - then you won’t miss any music shuttling back to camp for another layer/drink/hat/meal/sunscreen/chair/etc. Bring a large backpack for your trips to town/fest, one that you can strap your festival chairs/tarps to. Be the llama.
You’ll be fine for camping spots at Illium on Wednesday, so long as you’re not looking for your own back 40 acres. Circle the wagons and camp close to your new best friends.
And FYI, the festival food is maybe the best ever. You’d miss an entire set or more going back to camp for lunch or dinner.
Otherwise, prepare to have the time of your lives!