One thing you’ll need to do on the rails side is add those user/tag relations to the serializer so that those data are available to the front end. Look for some other plugins that call add_to_serializer or search for it in the discourse source, as it’s well documented there.
Then you’ll be able to use a plugin outlet in ember to add that to the page. The theme component documentation has examples of that.
I would probably add a route to manage the custom user fields rather than use a csv file, but if you want to go the csv way, you might want a job that reads it every hour or something, so find a plugin that has something in jobs/scheduled.
There is a Discourse repo called all_the_plugins that I sometimes grep to find examples of stuff. If you wade through github.com/discourse you should be able to find it.