There is a thread discussing how to get a list of installed Plug-ins
Since you were using the Plugin Manager Enhanced (good choice, by the way), the new_addons.txt file should be in the old User Directory. You can use it as a checklist for installing the 6.0 versions via the Addon Manager. (Those versions, at the very least, have different .gpr.py files with different versioning and additional attributes.) This will allow you to identify the addons that will have to be manually installed.
The other option (of taking an active reading of the installed plugins with SuperTool) would require re-installing a 5.1.x version.
The Plugin Manager Enhanced DOES exist for Gramps 6.0 installation. (Here’s the .gpr.py file for it so that you can see the name and category. And its listing in the manifest for the Addon Manager.)