There are the mergerfs tools which offer a tool to balance drives. You'd install the drive, run the balance tool, then use as normal. Or you use the rand policy.
# mergerfs.balance /mediaand it will balance out everything on the pool that is in the relative path folder "/media"? And rsync package is needed to be installed I see.
Ok, you can spread out the newly written data by using and change to "random" policy instead, that will help a little of course, but it will not move any old data on the drives.
"MergerFS timeout", how often will mergerfs scan the pool?