Ability to fetch all secret engines under seperate namespace

I would like to know if there is an ability to fetch all secret engines configured under each namespace while logged in as root via API

If yes, do we need to modify the policy to accommodate that or is there a separate API for it

You would need to use the sys/namespaces/ API to discover namespaces, and recursively descent through them, calling the sys/mounts and sys/namespaces/ APIs in each child namespace.

You would need policy which allows you to do this explicitly.