If you wish to have an independent photo archive in iCloud (or other storage location) you can create a folder in iCloud storage using the Files App. From the photos App, you can then use the Share Sheet to selectively save Photos to this folder.
These photos will not be visible using the Photos App - but will, while still being stored in iCloud storage, be fully independent of iCloud photos. After duplicating your photos to this folder, the original photos can be deleted from your Photo Library - and iCloud Photos.
An easier method in achieving the same goal would be to use a third-party File Manager App. Some can easily access and manage files and photos between a variety of external storage options - including iCloud, third-party Cloud services, external media devices and network storage. One App that comes readily to mind is FileBrowser Professional by Stratospherix:
https://apps.apple.com/gb/app/filebrowser-professional/id854618029
You would be well advised to review the extensive online Manual and help files to determine suitability for your needs.
https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php