Recommendations for SATA expansion?

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I have also seen some NVMe SATA expanders, which are alluring, however my OS drive is a 1 TB Samsung 980 Pro and the other NVMe slot is labeled in the specs as "CPU" which...I don't understand. I also have zero idea whether these expansion cards are too good to be true.

I want to circle back to this for a quick explanation.  Consumer CPUs only have enough PCIe lanes to directly attach one NVMe drive to the CPU.  As in no middle man.  The second NVMe slot on your board goes through the chipset and shares bandwidth with all the other connected devices.  Like USB, SATA, audio, etc.

The "CPU" NVMe slot is usually on the board between the CPU socket and first PCIe 16x slot.  Practically speaking you are almost certainly not going to notice a difference for normal usage.  If your main NVMe SSD is currently in the lower slot that goes through the chipset you should move it to the upper slot.

Going back to the HBA card, if you do choose to go with the M.2 to SATA route, make sure it is an M keyed adapter and not a B keyed adapter.  What's the difference?  M keyed means it is a true PCIe to SATA bridge.  B keyed means the M.2 slot is acting as a single SATA interface that is being split, aka all the drives share a single SATA interface's bandwidth.

Here is a M keyed M.2 to SATA adapter.

https://www.amazon.com/Adapter-SATA3-0-N...B0B5RJHYFD

This one says it is PCIe 2x.  Which shouldn't be a problem.  It wouldn't matter anyway since most secondary NVMe slots are 2x anyway.

Really appreciate the explanation. Are there performance disadvantages to going the NVMe route? Seems like possible greater throughput but unknown/unproven tech. Whereas the PCIe x1 card is proven, but limited based on the PCI slots available. I'm essentially pulling all the guts out for this new case, so I have plenty of time to get things correct...