This will monitor workspace changes and update the current workspace on other monitors. The idea is that workspace number 1 is paired with workspace 11 on second monitor, workspace 21 on 3rd monitor...
run it in background along your sway session
Problem: Sway does not allow switching workspaces without focusing it, and focusing a workspaces moves the mouse cursor. So it's impossible to change the current workspace on the secondary output without disrupting the user.
As a consequence, the implementation - for now - will only update the workspace when the moves moves to the output, but this is far from ideal.