Caching does not generally improve performance. Once the cache is full, normal read/write speeds are what you deal with. The only time caching improves anything is if you are accessing a large number of small files. But in most cases, it really will not help and in fact will slow things down.
So what you do is you set up the 2 NVME drives in a mirror configuration as the “system” drive. The “system” drive is not the “OS” drive as the OS is spread through all the drives. But it is where the default storage are for your apps and you can use it for things like Virtual Machine hard drives, etc. Apps will launch more quickly off the SSD drives, etc.
Here I disagree with you CACHE is intended to keep all the most requested data in one place and relieve the HDD disk of operational requests. Normally, if CACHE fills up, then the mechanism should be closed when it is automatically cleaned.
QTier, on the other hand, is intended to store the most requested data and offer it to the client from nvme Disks.
The difference between CACHE and Qtier is that CACHE duplicates data, while QTier transfers existing data from cold disks to active disks.
That is the theroy(for reoccuring reads or short writes), but sadly not working right, as multiple cache mechanisms, that QNAP has tried over the years, do not work right (e.g. do not destage dirty blocks)
Maybe YES you are right, but I see how Tiering Working in Other Vendor systems, and performance Increased significantly, when need more faster get or put data to NAS