Paired with a $25 Coral M.2 Accelerator [1] this is actually an interesting alternative. Twice the price of the Coral Dev Board Mini, but much better hardware.

1: https://coral.ai/products/m2-accelerator-bm

Neat. Looks pretty similar to the ODROID H2+. Some differences I see: the Odyssey has soldered-on RAM and eMMC, the extra M.2 slot, fewer USB3 ports, and 1 GbE rather than 2.5 GbE. I think they're similar in price (once you add the missing RAM and storage to the ODROID H2+).

Is 110x110mm a standard form factor? Are there cases available that could fit one or two 3.5" HDDs?

I wanted it look pretty, so i went with cheap slim brushed aluminium miniITX box. Fixing MB, notebook PSU and disks will be custom job.

Note that one m.2 is key M, and the other is key B.

Almost all NVMe SSDs are key M.

J4105 have total of 6 PCIe lanes, thats probably why. It'll still take SATA SSD in second M.2.