I got it working by using Preparing Assets for Chunking | Unreal Engine 4.27 Documentation to generate the pak files and Pak Loader Plugin in Code Plugins - UE Marketplace to load them. Well worth the money to avoid the pain of figuring that out myself.
This is how I load a map and a blueprint.
In 5.1 I additionally needed to check the map used in the extra chunk for cooking under Cooked Maps in the Project Launcher for it to be in the pak (worked in 4.27 without checking).