I think the easiest way to get 10 GbE on ITX in a case, is with a little modding and doing a m.2 x4 PCIE to regular X4 PCIE and adding a PCI-E 10 GbE card using the rear m.2 slots on motherboards to keep aesthetic clean.
For now, I think you're right. This ought to work well with recent dual m.2 Intel boards, at the very least. Does anyone know how a 10GbE (or at least 5GbE) NIC performs when connected through an X470/B450 chipset? Not that any AM4 ITX boards currently have dual m.2 anyhow, but it would be an option if they existed. Also, are there any m.2-to-PCIe risers that don't have the slot directly attached to the PCB, but run a flex cable of some sort from the m.2 to a secondary PCB for the slot? That would sure be far more convenient than needing to fit the rather tall slot+a riser cable between the motherboard and the tray.
Mod Edit: Post edited to comply with community standards.
Right, posts edited to comply with community standards, thread reopened.
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The easiest way, by far is to get a Thunderbolt 3 capable board, the ASRock Z270 and Z370 Gaming-ITX/ac comes to mind -- and while last November I suggested an enclosure + PCIe card now I can suggest the 180 USD Sonnet Solo. Mischief managed. No hacks, no risers, no problems. At 0.2L perhaps it's small enough for our esteemed forum even.
Eventually someone will take that IC, mount itt on an m.2 card and run some sort of cable from it. The Aquantia AQC-107S is just 12 mm x 14 mm so it should fit.
Neat thread, good info. I added a 10GbE card to my media box using an Era Adapter M.2 to PCIe x4 extender. I got the "R42SR" version. It mounts to the bottom M.2 of an Asus B350-I motherboard and works just fine.