Download all 155,000 original photos from iCloud to an external hard drive.... 1000 photos a a time ??

First increase the hard drive space Cloud and Hard Drive - Apple Community

You can select maximum 1000 photos at a time and choose most compatible ( including edits ) from iCloud.com and click on download .

Download iCloud photos and videos - Apple Support

A second method is export photos from system photo library to a desired location say download folder in finder and connect an external hard drive to the Mac , then transfer the photos from download folders into the external hard drive , hence all your photo data is saved .

You can select few photos see this article Select and deselect photos in Photos on Mac - Apple Support and delete to recently deleted folder see this article Delete photos or recover deleted ones in Photos on Mac - Apple Support ( its your choice to delete them permanently immediately or after 30 days the photos will be removed from the iCloud server ) .

In this way the complete photo data is still on an external hard drive , but the space is reduced in photo library .

See this article to export photos Export photos, videos, and slideshows from Photos on Mac - Apple Support

Note : there is no way to select more than 1000 photos , the photo application will hang , you have to perform the steps multiple times , in case if icloud storage is low upgrade the storage plan Upgrade to iCloud+ - Apple Support

Read this article also Create additional photo libraries in Photos on Mac - Apple Support

How to restore photolibrary from Icloud - Apple Community

This is probably one the reasons why I am occasionally have heard the recommendation from Apple employees at the Genie bar to keep Photos Libraries small, not larger than 20 000 items at most.

What you could try is to create an additional Photos Library on an external drive with enough storage to avoid "Optimize" and make this library the system Photos Library and then the iCloud Photos Library. You have to do this in a different user account, not your main user account where you are keeping the optimized library, as we can have only one System photos Library in our user account, and it would be too expensive to switch iCloud Photos temporarily to a different Photos Library, as you would have to sync the library again after switching back to your internal library.

I use a second user account an older Mac that is never moved or carried around to download all originals to be able to back them up, so I do not have to unplug the external drive - a system Photos Library needs always to be available for the background processes. Then you can export the originals from this library.

Photos can’t be transferred directly from iCloud to an external hard drive for that you need to set up iCloud photo library see the article Set up and use iCloud Photos - Apple 

And its preferences Change preferences in Photos on Mac - Apple  open finder > Go > home > your user name > pictures either drag and drop Photos Library.photoslibrary to an external hard drive , the second method is right click on Photos Library.photoslibrary and click on show package contents in the upcoming column , you can also drag and drop complete masters folder to an external hard drive that is connected .

Also as per the article Back up the library in Photos on Mac - Apple 

Manually copy your library to an external storage device: Drag the Photos library (by default in the Pictures folder on your Mac) to your storage device to create a copy.See this article also

Photos library - Move your Library to an external storage device

Yes, it would be much more user friendly. It is easier to state that we want to download all files than having to select 1000 images that have not yet been downloaded.

Apple is currently announcing a new version of iCloud.com. Perhaps this will make it easier to download files.

Have you also seen this announcement?