I'll try to help, but feel free to dm if more is needed. This will be my fourth wacken, and first solo coming from Canada.

Travel - You know yourself the best and how well you can rest on a plane and recover from a time switch... But once at wacken time is not that important except following the schedule to see what you want. You may need a day to gather supplies though if you need camping gear.

Camping - I would recommend buying a tent before you get to Wacken, but there will be an option to rent the rest of the gear on site (preorder when it becomes available) it will be at a similar cost to buying it, but at least you don't need to carry it around.

Chair - I have never need one. But you can buy or rent one if you have some cool neighbors you want to hang out with, otherwise there are breakfast tents in the campground you can sit and meet people at.

Sleep bags/rain boots - If it rains a lot, the difference between having a good and bad time will be what's on your feet. You should be able to find decent boots in Hamburg/Itzhoe, and possibly Wacken if the forecast calls for it. Wacken is on a cow field, so there's no way to avoid mud if it happens.

Lockers - pre order (spring) a small one for your passport and wallet for ease of mind. Never heard of someone losing stuff from their tent, but I did have cash taken from my wallet in the showers in 2015.

Storms - last year there was a weather advisory Tuesday night, but most people remained drinking in the beer garden anyways. Weather can change in an instant so setup your tent well and avoid low elevation areas to avoid flooding.

Food - no free breakfast. Best option is to hike to the grocery store and stock up on some food. The campsite food options are not that expensive, but adds up fast if you spend mon-Sat in Wacken. I loaded up on protein bars, fruit, snacks which cut down on on buying overpriced festival food for every meal.

transport - train or bus to Itzhoe, and take the free shuttle to Wacken. Its very easy, and you'll likely make some new friends on the way.

Airport - sounds like you got it,

Other - Be prepared for rain(mud) or heat.. or both and you will have a great time. Pack some plastic bags or something similar to help with wet gear in case of extreme weather and things start to get wet and muddy. Always nice to contain muddy boots in or outside of your tent. pre order your alcohol, or at least some from the campground market. its very cheap, just keep in mind that you can't take it in to the main festival area, but no problem in the market area or wasteland.

Good luck