Hello,
phpdocx Premium licenses support using in-memory DOCX documents. On https://www.phpdocx.com/documentation/cookbook/in-memory-docx-documents you can read all information about this feature.
Please note that in-memory DOCX documents are not DOCX files as strings, but DOCXStructure objects (that can be serialized if needed). In your case, you need to generate the DOCX documents as files to read them with CreateDocxFromTemplate or create in-memory DOCX documents in the case of using a Premium license.
Regards.