What you’re trying to do will currently taking some hacking - there is no straightforward way to do it.
I think there is a legit feature request in here - to allow tags to be configured with an emoji and also to allow determining whether both tag text and tag emoji are displayed, or just tag emoji.
There is a theme component for allowing tags to get icons, but there is no way currently to suppress the tag text. Maybe this could be added to the component as an an option?