As you can see from the error message posted in your screenshot, you cannot shrink the size or number of disks once added to an aggregate without degrading the aggregate. The only way to move a disk from an existing aggregate is to first fail a disk from the working aggregate, manually unfail the desired disk, zero the disk, then add it to the desired aggregate. Again, keep in mind that the disk you removed from the existing aggregate will cause the aggregate to now be degraded.
The command "storage pool show" is specifically for Storage Pools, which is a performance-enhancing feature and you need to use the command "storage disk show" to view disks and specifically "storage disk show -spare" to view the available spares in a Cluster. From the error in your output, it appears you do not have two available spares to add.
Here is reference documentation that provides more details for the "storage disk" command: