How to install Qnap QTS OS on NVME M.2 SSD
If you want to install OS on the NVMe, you should remove HDDs and keep only NVMe SSDs in there. The during the setup (will need to reset your NAS to factory defaults) you will be able to create volumes and RAID on a pair of NVMe M.2 SSDs (or 2.5inch SSDs). The system will then be installed on this volume.
Afterwards, you can put inside HDDs and build a RAID for these mechanical drives. Now all OS files will live on SSD also default app installation location will be NVMe. Alternatively, you can create NVMe volume and kee HDD RAID as the main storage location. All apps can be migrated to NVMe storage.
I hope this helps.