Overall, version 6 works well, but I have a problem that QNAP support confirmed:
You can create a “new pool” that uses Qtier.
==> But you can’t migrate an existing pool ZFS to Qtier ZFS (like on QTS with ext4).
Support told me that the migration feature isn’t yet available in the beta but is on the roadmap. (They weren’t certain about its availability in the RC/Final version of V6.)
The migration for Virtualization & Containers went smoothly on my TS873A.