Following Metadata should be accessible from PHAsset Object: Title, caption, Keywords. Write now they are available in the SQLite Photo Library, but thats is not a clean solution.
In the latest 27 beta SDK you will find a new API that provides these: PHAssetExtendedMetadata
You can use this API to fetch the caption, keywords, and original filename for a PHAsset. For caption and keywords there are functions on PHAssetChangeRequest which let you modify them.
More information here: https://developer.apple.com/documentation/photos/phassetextendedmetadata
