Backup/Archive with Lightroom Classic from cloud workflow?

Yes, you can sync a Lightroom Classic catalog with your cloud account, and it will download a copy of every image in the cloud. TIP: before turning sync on in the LrC catalog, visit the LrC Preferences>Lightroom Sync tab and there you will see an option to change the default local location for the downloaded files....the default is an obscure location in AppData>Local, so I do suggest you set a different location (and as you're over 1TB of data in the cloud you might need to be setting the location to an external drive, or an internal drive that is big enough to cope). On that same Preferences tab there is also the option to choose to have the images stored in one of the standard Lightroom capture date-based folder structures. Again, worth doing as you really don't want 1TB of images in a single folder.

Once they have all downloaded into Classic, you can start removing images from the cloud. The best way to do that is actually in Classic....identify which images you want to remove from the cloud, select them all and put them into a non-synced collection first, so that you can easily find them after removing them from the cloud. Then with them still selected click on the All Synced Photographs special collection in the Catalog panel, right-click on any one of the selected images and choose "Remove from All Synced Photographs" from the resulting menu. That will remove them from that ASP collection and in doing so it also removes them from the cloud. 

Once that is completed (check in the Lightroom desktop app to ensure they've been moved to the Deleted album), you can then go to the collection that you created in LrC to hold the images being removed from the cloud....if you want to resync them as smart previews back to the cloud (so not counting against your cloud allowance) then either enable the collection to sync (so all the images in that collection will be uploaded as SPs) or if you prefer to only sync some of them back to the cloud you can select the ones you want and manually add them to that All Synced Photographs special collection again.

Just understand that syncing FROM Classic is entirely selective, not automatic, so if you want to do that you have to enable it yourself as outlined. Once synced, of course, edits can be done in any of the apps and will sync to all the others, including LrC. 

One word of caution....keywords and location data and presets do not sync between Cloud and Classic, so that initial download into Classic will not bring any such data down.