You could try %frontend_asset_updated%, this should disregard date updated for any changes made to paint layout. However it won’t work for metadata IF the metadata value is update to the current asset/content.
Would be good to have a keyword like that actually, as this has come up a few times now.
Using published date and going through safe edit is a good idea.
Alternatively you could have a metadata date field that updates automatically by a trigger everytime content or metadata is updated on an asset.