What slots can I use for Intel Optane memory on my TrueNAS server?

Hi folks, my NAS is running on this motherboard: https://rog.asus.com/motherboards/rog-crosshair/rog-crosshair-vi-extreme-model/spec/

I currently have the following:

• M.2 NVME Boot Drive 1TB SSD
• M.2 NVME App Drive SSD
• SATA HDD Pools
• SATA SSD Pools
• 2x GTX 1080s SLI
• 1 USB 3.1 PCIe card
• 32 GB DDR4 (wish I had bought 64Gb before prices went crazy, but it's perfectly manageable)

As you can probably tell, this was a gaming PC at one point, but now I have it in a massive case, so it's really easy to get in there and mess with stuff on my home rig.

Here's my quandry: I currently have a 1TB M.2 drive as my boot drive of which ~16Gb is used. SSDs are obscenely expensive right now and I'm out of room for games. I've read online that you can use Intel Optane memory as a boot drive.

This unit in particularly was recommended: https://www.amazon.com/Intel-Optane-Memory-Xpoint-Retail/dp/B078ZCZ5H9

My question is this, and it sounds a bit silly for someone who can self host to not know this, but how do I insert the optane memory and is it compatible with my mobo? I see it has a PCI 3.0 connection type but it has a little half-moon screw divot on the end like an M.2 drive. Do I just push a drive like this upright into one of the PCIe slots or does it have to go into that second m.2 slot the motherboard specs show supports PCI 3.0. I'm trying to avoid using the second M.2 slot since I might want to add a second SSD for apps later on as I already get an earful whenever the Jellyfin goes down. :D Alternatively, I'm considering using the SATA SSD as a boot drive. I've read that once everything is up, it's all loaded into RAM anyway and the boot drive speed is not really important anymore. I can tolerate an extra 15 seconds on startup more a machine that might get rebooted once or twice a week when things are running smoothly. I could give up the USB 3.1 PCIe card as I think there are some headers hooked up to the case ports.

What would you do in my situation?