Hi Oliver, interesting question! You are correct, on many small devices like a Raspberry Pi, you could just flash lots and lots of SD Cards (we are building the EtcherPro for that!) and boot them all up in bulk, and be done!
In the x86 world though, we typically build an install image that is flashed to a USB stick. When booting from the USB stick, the machine powers on, boots from USB, and the contents of that USB stick are copied over to the onboard storage like an SSD or NVME drive. So, you could flash lots of USB sticks, or, perhaps you could place that image file on a network location and have the NUCs (or other x86 machines) boot from network? If they all go and attempt to boot from that same image, they will power on, boot from that image file over the network, copy the contents to SSD, then set boot options back to standard (local) boot. Perhaps that would help speed you up.