Yeah, you’re running into the CustomGPTs Context Window.
If you need a user other than yourself to create with this, then I think you’re pretty stuck with the ChatGPT User Interface until you’re comfortable building an API Assistant with a friendly UI of your own.
The Context Window is pretty artificial and grows all the time, as it just did with the release of 4o. So you’re CustomGPT will be able to do it all in one go sooner or later, maybe even now once the new model is fully released. You have more control over this with the API—as for example knowing exactly how much of the Window you’re using for a prompt.
Meanwhile, you’ll have to break your project into smaller chunks with multiple responses required to fill the spreadsheet.
For creating Actions that link directly to your Google Sheet, see this thread. It’s pretty dope.