What’s the best method to use this is sequencer when rendering a cinematic?
Just make sure the level sequence is in your level (not just an asset in the content browser) and Cesium for Unreal should automatically wait for all the data for each frame to load before allowing the frame to be snapped.
Is there way to take the tile sets offline?
No, not really. You can license Cesium World Terrain and Sentinel-2 imagery from us for local use, if that helps. But if you want Bing Maps satellite imagery or Google Photorealistic 3D Tiles, it’s a violation of the terms of use of both of those to download them for offline use.