Knowing that since some time BUT unfortunately, this specific MB doesn't have a GPU slot and therefore, you cannot install a GPU on it.
What you could try, is to update its BIOS, set it up as for "production" with all BIOS settings, pull out the HDD/SSD used for installing ESX, and do the installation on another computer where you have a GPU.
After ESX is installed, you can migrate the HDD/SSD back to the NAS and boot as usual.
For me it worked just fine with this procedure but anyhow I like HyperV a lot more and I stick to it as it offers me more options.
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Why do you say it doesn't have a GPU slot ? PCIe 2.0 x1 can't be used ?
by the way, salutari tovarase Bucurestean