There are two types of breakout for sata to sas cables, forward and reverse.If you use sas port on mobo or sas controller you need to use forward breakout cables.If you want to connect backplane to sata ports on mobo or some cheap marvell pcie card with sata ports you need to buy reverse breakout cables.
For FreeNAS it is commonly recommended to use an LSI host bus adapter. These days LSI are now Broadcom, but that is harder to search for.
An appropriate card for your situation would be along the lines of a 9211-8i, where the 8i means 8 internal channels.
You can easily find these cards used, or if you want to save even more you can flash rebadged cards, for example: https://www.servethehome.com/ibm-serveraid-m1015-part-4/
Make sure when you get the card that it is flashed with IT firmware, not IR. You can flash the correct firmware (and probably want to update to the latest firmware anyway).
edit: Just checked out the Silverstone card you linked and it does use the LSI2008 controller, which is the same controller used by the LSI 9211-8i and IBM ServeRAID m1050, so that would work. The IBM cards are very cheap on ebay though.
What i was trying to point out is that you might have used wrong type of breakout cables for the job. Reverse breakout ones should have been used in a first place with backplane. try ordering proper cables before getting more expensive pcie card.They look the same but a wired different ways and work different ways.
Cheers, I will buy reverse breakout cables for the first two rows of hard drives and then an LSI 9211-8i, for the other 2 row of hard drives.
It would be great if the community could make a FAQ Questions about servers and cabling. It’s not until you go out looking for this information that you realise how hard it is to get.