Thank you. So storing any Google docs, sheets or whatever in iCloud in the expectation of being able to open and work on them on an iPad Pro is not possible, they have to be stored in Google Drive to do so.
That is correct. Google Docs only supports it's own services for working on documents. It is sometimes possible (if not easy) to store the document somewhere else, but to actually work on it you have to put it back in Google Docs, which will store/sync the document in Google Drive.
While it is possible to work with a Google document offline, it will always sync back to Google Drive when the opportunity arises. In some ways Google's office apps are the opposite of most other productivty apps. Pages, Word, and other's all originated as offline apps. The ability to store and work on documents in the cloud (online) was added later. Google's apps originated as online apps, completely dependent on your internet connection. The ability to work offline was added later, as a fix for those who needed to work on documents when there was (temporarily) no internet connection available.
As a result, Google's services and apps tend to be quite versatile and powerful, as long as you have an internet connection. However, they are only so-so when you don't have the internet, and offline support is only an afterthought.
Basicaly, if you're going to use Google Docs then use them in Google's enviroment. Any attempt to split Google's apps from Google's servies and storage is going to be difficult; because Google doesn't support it; and probably never will.
They are, of course, a lot more flexible when it comes to letting you use Google drive with other apps.
At this time, there is no 'one' cloud service that will do-it-all. If you have any sort of variety in your apps, you're going to find that you need more than one cloud service to get the best use of them. Fortunately, basic storage limits for most cloud services are free, or inexpensive.