Mini server out of 13" motherboard

This sounds tempting. However, it’s surprisingly hard to find a definitive answer on what works and what does not. For instance, this thread says that the secondary M.2 slot is E keyed, and based on my understanding, this rules out all kind of storage devices (both mSATA and NVMe.) But I’m not an expert here.

I would be grateful for some examples. So far, I coudl only find A+E keyed cards that convert to either USB 2.0, PCIe x1 or Mini-PCIe. None of that seems ideal.