It would be great to have this feature integrated.
I have a lot of smaller disks (sub 4TB) that I'm working on removing from the array. And I have no need to replace every single disk with a larger one.
What I would love to see is the possibility to mark disks for removal. And during the next few days the data on the disk will be reallocated to the free space on other disks.
Then you can just remove the disk from the array.