Export all metadata

I guess your may use the Report Builder from ACS commons. It allows you export data into the excel file:

https://adobe-consulting-services.github.io/acs-aem-commons/features/report-builder/index.html

You will have to extend it with a custom report type, so it will handle assets metadata, please follow https://adobe-consulting-services.github.io/acs-aem-commons/features/report-builder/extending.html

Export delta

1. Run report programmically, save the excel file in the filesystem or /bin folder of AEM

2. Update the Upload asset workflow, so it will append metadata to that file

3. Add remove listener, so it will remove the record from file

4. Implement API to download excel file from file system or /bin folder****