Hi @Shock, welcome to the community!
There’s no built-in way to do that. The only way I can think of to do it would be a little tricky. While on your loading screen, you could add a virtual camera to the CesiumCameraManager. Move that camera along your flight path in small steps, at each step waiting until loading is 100% complete. As long as your Maximum Cache Bytes property on the tileset is large enough, you should then by able to let the user fly that same path, and be sure all necessary tiles will already be loaded.
Also, when you first released Cesium for Unreal, you mentioned that you were planning to support downloading data for offline / packaged use. Did this functionality ever get implemented?
Yes, Cesium ion now allows you to clip out areas of tilesets and download them for offline use. It’s also possible to license entire tilesets from us for use offline. Unfortunately, this does not include Google Photorealistic 3D Tiles or Bing Maps, due to Google’s and Microsoft’s licensing restrictions.