Hello,
phpdocx Premium licenses can generate a DOCX as a stream using the included ZipStream class, but this class doesn't allow adding streams to generate a new ZIP file that includes them (ZipStream uses gzcompress that doesn't allow adding streams/objects). phpdocx doesn't include a class to generate a ZIP stream that include DOCX streams.
PHP ZipArchive doesn't support adding streams (only files and strings), so an external library or tool should be used. Maybe ZipStream-PHP (https://github.com/maennchen/ZipStream-PHP) is a good solution, you can generate DOCX streams using phpdocx and using that library you could generate a new ZIP from these strings. Or you can use a RAM disk (https://www.jamescoyle.net/how-to/943-create-a-ram-disk-in-linux) to avoid generating the DOCX in the filesystem.
Regards.