I'm a big fan of cold zink spray for rust prevention it's self healing, years ago I made doors for oil rigs in the North sea and the paint we used was zinc rich you literally added a bag of zinc powder to the paint, my point is if it's good enough for the North sea environment it should last on the road
If money is not too important check out Retro power on YouTube they hot zinc spray the underside then seal every joint then use raptor bed liner
https://youtu.be/JOx_HOwzz3E