What I do is on the originator drive, I create new subdirectories and start categorizing items by content; like I'll put all the ebooks into one directory, and all the television into another. It just makes it easier for me to find things later if I can just head to the drive with all the television on it.
If there's a particular directory with a lot of content, I might create further divisions - maybe shows that are finished vs those who are still getting new episodes, or sitcoms vs drama, that kind of thing.
Then I make a list of how big each master directory is, and I start copying them over to the most appropriate-sized drive. I usually find that I can fit in one large directory, and a couple of smaller ones, and then the last drive gets all the leftovers. I also tape a post-it note to each drive saying something like "2022-23 television" or "science fiction audiobooks" or whatever.
I also create a new directory on the originating drive called something like ++COPIED and, once I've copied content to a new drive, I move the original directory to ++COPIED: I'll still have access if I need it, but I don't have to keep track of it any longer. Once everything is successfully copied over, I can just delete that one directory.
It's a manual process, yes, but it does make it easier for me to find stuff when I want to look at it again later.