Some time ago, I had written about my adventures in dealing with the 5,000 item limit in an Office 365 SharePoint Document Library. In troubleshooting and working through this issue, I ended up with a SharePoint Document Library folder with more than 5,000 items in it.
The only option to clean up this library was to reduce the number of items to below 5,000. To accomplish this, I created a Non-Folder view and individually selected and remove documents. Given the number of documents, this was a slow and time consuming process - a true effort in futility. Yet, I could not delete the Document Library, so I was just stuck with it.
Enter ShareGate. I was able to utilize the ShareGate tool to Move my documents from one Document Library to another within Office 365. This saved me a ton of time and allows me to be able to once again work with my site on another possible solution.
There are other tools that do the same thing. However, I wanted to submit this alternative for getting a Document Library back to working order. If you do not wish to purchase software, my company as well as other companies that participate in the Office 365 Grid are available to assist in migrating data.