You can’t set a campaign to be the default layout you can set only a single layout as default layout.
The default layout is there for error conditions, so when nothing else is scheduled, or when whatever is scheduled is invalid for some reason, it will show the default layout, It’s not designed to hold content.
You should be scheduling the content you actually want to show with the scheduler.
As for you request, the default layout will not be changed. It’s not how the product is designed to work.
Spring Signage - Support Engineer