If you’re using RAID 1, you could move both of your disks to the new device and follow the steps of “replacing disks one-by-one” to replace the disks with the new ones. It may be risky so you’d better have backups before that, but you’ll keep all bytes of your current files as-is after the RAID rebuild.
If you’d like to copy, USB will not work because it needs one side to be “host” and the other side to be “client” but QNAP servers will only act as host. You could connect both devices to your home network and use HBS to sync using “remote NAS” or “remote rsync” as sync source.
Even it’s a one-time job, syncing is better than copying because it could continue easily if interrupted.