New config references the parity disk, it allows rearranging array data disks and recalculating parity based on the new arrangement.
It doesn't erase any data disks, only the parity disk(s).
If you want to wipe a disk, you need to format it, the easiest way to do this in the Unraid GUI is to click on the disk and change the desired format type, start the array, and select the format button after making sure only the disks you want erased are listed and checking the "Yes I want to do this" button.
Array disks can't be trimmed, typically you would use various pools for SSD's. Currently Unraid requires disk1 in the array to be populated to start, so you can use any old USB flash drive as disk1.
You can assign specific duties to various pools.