Photos export full res with Optimise Mac Storage?

The full sized picture will be loaded before the export. That's really the point of Optimize-- Photos keeps smaller preview images (not just thumbnail) locally, but Photos downloads what you need when you need it. However, the downloaded original may be offloaded immediately after it's used.

The real concern is for backup, since Time Machine doesn't force a download. For backup, see this:

Backup iCloud Photos with an Optimized Mac - Apple Community

MT1203 wrote:

I am a bit confused again. Are you saying that the "File - Export - Export Unmodified Originals" won't actually pull down the original file unless "Downloads to original" is enabled instead of "optimise. Thanks.

As Richard said, "File - Export - Export Unmodified Originals" works, but the originals may be offloaded immediately after it's used, if there is not enough space on the volume where the library is stored.

...so sorry for the unnecessary confusion -- I was just lamenting that the CCC iCloud Drive backup option does not work with Photos.

BTW Carbon Copy Cloner has an option to temporarily download, back up, and evict optimized iCloud Drive files.

But sadly that does not cover optimized iCloud Photos:

"Unlike the cloud-syncing solutions noted above (including iCloud), iCloud Photos does not offer functionality to download the full-resolution versions of these files temporarily, nor does it use cloud-only placeholders so CCC can not tell that a given file is not full resolution. If you would like to make a local backup of your full-resolution photos, select the Download Originals to this Mac storage option in Photos Settings > iCloud."

https://support.bombich.com/hc/en-us/articles/20686419951767-Backing-up-the-content-of-cloud-storage-volumes

Matti Haveri wrote: "File - Export - Export Unmodified Originals" works,

Also File>Export>Export nn Photos.

So, if all you want to do is export, it's no problem. But you said you wanted a backup, and ideally that would mean that you wanted to backup the database. Export doesn't do that-- it uses the database to produce some version of your editing. Then you would need to export both the original as well as a version of your editing. That will take a really long time, and you don't get the database.

The only good solution I see is the one I suggested in the Link, earlier.