You are unable to find Document Sets in SharePoint Online for Document Libraries that have been created
As of K2 Five and Cloud onwards, the workflow designer no longer has wizards that will allow you to utilise the Document Sets for your Document Libraries, you can however use SharePoint SmartObjects to achieve this.
1. I followed this article on how to create a document set https://sharepointmaven.com/document-sets-hidden-gem-sharepoint/
2. Then for the SharePoint Document Library that makes use of this Document Set Content Type, I had to regenerate it via the K2 App in SharePoint
3. In the K2 Designer, under your Site, there will be a new category called "Document Set" that will have a SmartObject available that contains all of these desired methods such as "Get Document Set/s, Create, Delete, Update, Copy, Move" etc that you can use to achieve this scenario in your use case.