If you want a good mini-ITX case, just buy a Node 202. You can use it horizontally and put your monitor on top of it.
Buy power-efficient components and load it with the thickest and biggest Noctua PWM fans possible, and set the fan curves low, and its basically totally silent.
I have a node 202 with water cooling that I built specifically to run Half Life Alyx at max specs while traveling for work. Fits in a larger ebags backpack very nicely with all the Valve Index kit. Now that I beat HL:A and am not traveling for work it sits under my TV and is pretty much a dedicated Satisfactory machine via steam controller.
Tangent—Coming out of hours of fantastical sci fi VR escapism into a nondescript hotel room in <industrial center, USA> is a very surreal and memorable feeling.
Buy power-efficient components and load it with the thickest and biggest Noctua PWM fans possible, and set the fan curves low, and its basically totally silent.