Did you ever get those local SSDs as copy-on-write overlays on Linux? I imagine it'd be easier with btrfs support for seeding device: https://btrfs.readthedocs.io/en/latest/Seeding-device.html

Yes, on Linux I was able to move the copy-on-write overlays to use local disks, which is one reason it performs much better (admittedly not a reason that would affect most people).

I am just using dm-snapshot for this -- block device level, no fancy filesystems.