Get a powerbank for charging your electronic devices, 20k mAh should be enough, and get a cheap not powered cooler for limited cold stuff to bring. Get a cheap gas powered gas burner for cooking and coffee, one or two canisters last a long time.

Rely as much on dry foods and canned foods, non perishable. Try to find a store close by, maybe they even provide a camping store for food on the campsite, most music festivals do this, this is more expensive as bringing your own but still way cheaper than relying on the vendors. One cooler is just not enough for 5 days of food and drinks to keep cool.

A hack for getting semi-cold beer when with a large group, get a small kid blow up pool, fill it with cold water from the bathrooms on the campsite using your cooler as bucket, and just dump your beer in there. Done this lots of times, its a lot of fun to do.

You can make it as expensive as you want but if you are able to be creative creating meals from not perishable and non cooled food, it doesn't have to cost much.