I just got a reply from Raptor Engineering.
One of the things I asked about was splitting the Talos II Lite's x8 PCI-E slot.

So:

>Has anyone attempted to use an expansion system (plx based or some other solution) that would break that X8 slot into multiple slots with fewer PCI-E lanes.

"Yes! We actually have several of these systems under test. If you're willing to work out the custom mounting arrangements, we can provide a list of hardware that could break that x8 slot to either two or four x4 Gen 3 slots"

Note, that mentions two x4 or four x4 Gen 3 slots.
I'm not sure if that means two x4 Gen 4 slots or four x4 Gen 3 slots, or if all options are Gen 3.

They did confirm that they are working on a reduced form factor board.