You seem to be under the impression that the “buckets” in this case are all or nothing. They are talking about partitioning the drives and raiding the partitions. The way he describes slowly moving data to an ever increasing raid array would most certainly work, as it is not all or nothing. These buckets have fully separate independent chambers in them that are adjustable at will. Makes leveling them possible, just tedious and risky.
I was at the make excuses stage until late last year when my excuses were fixed. Booted my windows install maybe four times since then, and that was mostly to grab files from it haha.