https://www.chinahao.com/product/557669060466/ those six are not USB ports! Those are PCIe x1 ports carried over the USB A cables to mining risers. I wonder whether somewhere in the dark depths of taobao a similar ITX board lurks...
I wish they still made those. I can't even find them secondhand.
Anyway, I've never seen mining boards in mITX. Not sure form factor would give any advantage; any enclosure for airflow would have to be large enough to contain the GPUs or at least allow for all the wires.
...although now I'm thinking of a small STB-like "mining box" where the USB ports are extended to the back. Who makes custom mobos? Maybe we're on to something.