Adat in/out would be the optimum route if you mainly use 2 inputs but sometimes need more. Babyface or Audient iD22.
The lynx stuff is superb, but is $2000 for 8 x 8. However, you'll be pro level for each of them. The USB works fine.
I'm happy w/ my TCK 48, but that is a FW unit. And they had lots of trouble getting the drivers right, which is probably why they haven't stuck usb on it. Shame, since it sounds really nice (esp. the DA, 12 x 10 + ADAT + spdif, top notch dsp effects).