You will want to stop any Docker containers that have a mapping to or within the share you are operating on while you make changes, aside from that though you dont need to make any changes since you are creating the subvolume with the original path. Likewise you shouldnt need to make any changes to share settings since so far as unRAID is concerned the new subvolume (which has the same path as the original user share) is the existing user share.
Where you store snapshots doesnt really matter, they can be anywhere within the same pool (they dont have to be within a subvolume). Snapshots themselves are just subvolumes anyway.
This is not entirely true but it requires some explanation. When you snapshot a subvolume the snapshot must be made somewhere on the same filesystem (pool) as it is a CoW copy of the subvolume (and a new subvolume itself). You can however send subvolumes from one BTRFS filesystem to another using btrfs send and receive (which are available in this plugin). Doing this copies the subvolume to the other filesystem and thus it is no longer CoW copy but a full copy taking up space on the other filesystem. Once a subvolume is sent to the other filesystem there is a way to send subsequent snapshots of that subvolume between the two filesystems in a way that maintains the CoW relationship between the subvolume and its snapshots.