I found a nifty trick with ESXi.
If you want to use an NVMe Drive as your main datastore, you can install to it as normal, but the Dell R720 will not boot to it. (at least in my case, using a Kingston A2000 NVMe SSD on an Orico PCIe Adapter card)
The UEFI simply doesnt see it as a drive.
But what you can do is create a 4GB HDD partition on the vFlash SD card, and install ESXi to that.
While smaller than the minimum disk size of 8GB, the installer will run correctly.
You can then reboot the system, and boot off the 4GB vFlash partition.
Somehow ESXi seems to realise what’s going on and switches over to the NVMe drive!
When it boots up, you will find that your datastore resides on the NVMe drive.