I do not think this is possible.
You cannot replicate pages/variants to other workspaces. Had my own share of experience having to do double work.
I think there's an unorthodox way of making your variant a "page template" and being able to click this with a new page, but this is undocumented and unsupported by ServiceNow.
What I did do in the past to prevent duplications of work in the future, is to create a "page collection" with the dashboard variants. Which can then be shared and used among other workspaces.
