Oki, well, seems like the option for NVME cache is useless 
Well, it depends on the data that you are writing. If it’s a lot of small files then the cache can improve things. In most cases, it doesn’t improve anything.
I think for Qtier start working well, I think you have to let it run for a while. The data used most frequently is stored in the SSDs and the data that is infrequent is moved to the HDs. The documentation says you will take a performance hit when it is moving data between disks. In your case, you may not have used it long enough to really start to see a difference for your most commonly used data. I don’t think it’s meant to act as a cache.
If you want fast performance on a storage pool, create a pool out of the SSDs. Store what you want accessed the most or where you need the speed (like virtual machines) in the SSDs. Let everything else be on the HDs.