Tiny gaming PC... by this i mean what is the best result you can get using a 2080 Ti (for example) in a LattePanda or Raspberry Pi connecting the graphics card via an M.2 (rasp has an add on board for m.2 i believe)... It'll only run at 4x rather than 16x however for a video you should still get some great results before CPU bottle necking and be able to make a nice super tiny housing.
Edit: after doing a bit of research you should be able to get 4K, 60 fps on high settings.
As an extension you could do the smallest build with best fps per £, perhaps with a 1050 Ti or something similar.