@Gohrilla : normally, when adding a drive to a storage pool, the "wizzard" will automaticly expand the volume with the available free space.
Sometimes you may have to manually expand it.
But, when using SHR, as can see in the Raid Calculator I linked to above, there are some things, due to SHR, that will limit the available space.
Currently you should have 19TB available space, 8TB beeing used as "protection" and 2TB are "wasted" unused space, all due to SHR.
(if we round up the numbers shown in your pic's, it all add's up, some space on each disk is used for system/swap).