Not exactly the same purpose but here is a small tool I wrote to group workspaces across all monitors : https://github.com/J3rome/i3-multimonitor-workspace

TLDR: global workspace span across all monitors. Changing workspace change for all monitors (Similar to what happen in windows & macos). In my opinion, it allows for a better separation between workspaces/tasks when working with multiple monitors.

It's an MVP/alpha software. Need some optimisations but it does the job (been using it daily for over a year now).