Backup/archive Messages in iCloud

I want a complete local backup of all my iMessages. There are several apps that allow you to backup iMessages, but only what already exists on a device, which is only be a subset of the complete set stored in iCloud. Neither iCloud backups nor computer backups include iMessages from iCloud . I have 16GB of messages according to iCloud, but my phone only reports 8GB. Is there a way to force my phone or (ideally) Mac to download the entire message history, so it can be archived?