That's the problem with "Optimize," isn't it! You could use File>Export>Export Unmodified Originals to send the originals to your hard drive. It may take quite a while, since Photos will have to download from iCloud in order to export, and then if will have to clear some and download more. I wouldn't try more than 1000 at a time.
Everything else that I can think of is even more complicated and cumbersome than the two you listed or using Export. This is why I don't use "Optimize" on my Mac.
I think you may have to face that fact that you need another external hard drive. You didn't tell us what sort of Mac you have, or even what madOS you're are using or what storage you have available. I have a MacBook, and I have chosen to put most of my files an a hard drive to make room for my Photos Library. I use a tiny 4GB SSD that weighs less than an ounce, so I don't mind carrying it along with my MacBook.
For a Desktop, the Library can easily be kept on a fast external drive. But for a portable, plugging and unplugging the Library that's pretty constantly trying to talk to the computer can be more problematic.
By the way, backing up by opening Originals and copying the contents is pretty awkward. For instance, their original filenames are kept in the Photos database, and you don't get that--or any other sort of order. You lose all the information in the database that represents your work in organizing your pictures! Most of us do Time Machine (or equivalent) incremental backups to one drive, and we periodically copy the entire Photos Library to another. But again, method works with "Optimize."
Also-- if you turn off "Optimize," that doesn't mean that suddenly all your originals get downloaded. They'll only be downloaded when needed. The best way to make that work is to create a new empty Library, set it as the System Library, connect it to iCloud, and let it full up.
On a Mac you can have many Libraries. Many of us solve the storage problem by having an Archive Library (or several) and a Favorites Library. The Favorites Library is the one connected to iCloud. Favorites is certainly smaller, and it might fit on the internal drive. The Archives can be stored on an external drive, and you just double click on one to access it.
