First, thanks for the interesting Optane videos.
For a fun new boot drive I just picked up a 960gb 905 and a u.2 to pcie card. The MSI z790 Ace offers many many choices for install. 2nd slot would be pcie 8x on the CPU 3rd slot would be 4x on the chipset. I could use the m.2 adapter and place it in a 4th gen m.2 slot direct to CPU without splitting the lanes of the 1st slot buy using the CPU connected 2nd slot.
So many choices. All likely good. What might be best?
Generally, you can use any “u.2 to pcie” card for a single drive in any slot. Bifurcation is required once a card contains more than a single drive.
What is the best option for your mobo?
I would try the third slot first. This will keep full bandwidth on the first slot, which most likely holds a dedicated GPU in your case. There are no bandwidth concerns because the chipset to CPU connection has much higher bandwidth than the Optane drive can muster (PCIe Gen3 x4).
NVMe drives (and Intel Optanes in particular) are all so quick that they benefit from a direct connection to CPU. This will improve latency, one of the key reasons to buy an Optane drive.
With the MSI board this comes at the cost of halfed bandwidth for the first PCIe slot.
Will you notice and benefit from the improved latency? Only you can tell.
Will you notice and suffer performance drops due to dropped bandwidth on the first PCIe slot? Only you can tell.