When you use the File menu : Save as Template..., one's eyes always train over to the blue highlighted, Add to Template Chooser button, and skip right over that other Save... button that allows you to put your templates elsewhere (e.g. iCloud Drive).

What I would do is make a new folder in the iCloud Drive : Pages folder by the name of User Templates. That is where you would use the Save... button from above to write new templates. There is no override to Pages coding that would tell it to use this new template location instead of its default that a New Document request would use.
The first time that you launch Pages after saving to the iCloud Drive : Pages : User Templates folder, it would open in that location, so you will have to be careful that your future default creation, and saves of documents don't get written into your iCloud Drive template folder by accident.
As for existing custom Pages v6.0.5 templates, they are located in your home Library folder. If you do not see that Library folder, then open a Finder window, click into your home directory, and press command+J to see its view options. You want to check Show Library Folder.
The entire path is the following, of which you can double-click through everything except the com.apple.iWork.Pages package folder. There you will have to right-click to Show Package Contents.
/Users/yourlogin/Library/Containers/com.apple.iWork.Pages/Data/Library/Applicati on Support/User Templates
I would select all of the User Templates, right-click on them, and choose Copy nn Items from the Finder contextual menu. Then hop out to iCloud Drive : Pages : User Templates, and with a right-click inside that folder, paste the templates into it. Do not move the originals.