Pannier space is finite and every gram matters when you are riding 200 miles a day in the heat. Lay everything out on the bed before you pack — then put half of it back.
The essentials are non-negotiable: documents, armour, water, tools and a first-aid kit. After that, favour multi-purpose kit and quick-dry fabrics. A fleece that doubles as a pillow, base layers that work hot and cold, and trainers you can walk in off the bike.
You will buy souvenirs. You will need the luggage space. Pack light, and you will thank yourself every single morning.
Somewhere over the previous two weeks we'd all accumulated considerably more Route 66 souvenirs than our suitcases had originally been designed to hold. Additional luggage became essential.