Unless there is an easy way to convert a DrivePool to mergerfs etc.

If I understood right, it’s just software joining together multiple NTFS drives. When you open in another os it’s just a normal NTFS drive

You could take two drives and create a raid1 (right? Or yolo without redundancy?) btrfs array. Move stuff from one drive, when it’s empty, format it as btrfs and add it to the array. Repeat for every drive.

But…

With this much data I wouldn’t risk if you don’t have experience with btrfs via cli