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Synology DiskStation DS1621: Is it possible to use NVMe SSDs instead HDDs as storage drives?

My brother just bought Synology DiskStation DS1621+ 6 Bay Desktop NAS Enclosure, and wants me to set it up for him.
He had not consulted me with purchasing it and I am tasked with getting it up and running.

He has bought NVMe drives just for cache and a separate 4TBx8 SSD NVMe drives for storing data.
From what I can find on the manual and searching the internet, this thing seems to be built for HDD drives and two NVMe drives strictly for cache.

Is it possible to use the SDD drives as storage drives in this Synology unit? If so is the use of additional SSD drives for cache redundant?

Any help would be greatly appreciated!