[Update] I played around with the plugin a bit more and I think I found it!
With my new Sage connected, I selected all of the ebooks on my device in Calibre. Then I clicked "Store/Restore current bookmark" in the plugin menu and then in the pop-up menu I chose "Restore" and ticked "Set Reading Status", unticking everything else.
Screenshot:
https://i.imgur.com/shz4bwJ.png
This seems to have worked like a charm! Now it really looks like I transferred my entire library from my Aura One to my Sage, with all metadata - including the custom columns - intact!
I apologize for not being more thorough in my investigation of this plugin. Signs of old age, perhaps?