The issue with that is it's A64 based, you only get one native 1Gbps port on that and you kinda need at least 2 if you're using a router for it's routing functionality.
You could get a rockpro64 and stick a nice network card in it's pci-e slot and probably outperform the clusterboard.
The Clusterboard solves that issue by including a built-in unmanaged switch (which is indeed how all 7 SOPINEs can share a single Ethernet port). So a router-capable Clusterboard seems like it'd just be a matter of adding more ports (be it by using the same switch chipset and swapping SOPINEs for ports or by using a different chipset with support for more than 8 ports - assuming that the chip in question is indeed limited to 8 ports).
Unfortunately I can't seem to find any detailed info on the Clusterboard components/schematics on their wiki, so I don't know what more it'd entail.
Ofcourse you can add a switch to it but that doesn't magically give you more routing bandwidth. You'd have to do router-on-a-stick which halves your bandwidth
Good point. And the SODIMM-based pinout doesn't seem to include support for PCI-E or additional Ethernet, so that is indeed a bit limiting (though I'm betting some of the pins could be repurposed if both ends are configured for it, e.g. if there was some way for a SOPINE and baseboard to negotiate "hey, neither of us have camera support, so let's use the camera pins for extra Ethernet").
You could get a rockpro64 and stick a nice network card in it's pci-e slot and probably outperform the clusterboard.