Would anyone consider using the 8mm as a...
Would anyone consider using the 8mm as a rappel line for canyoneering or caving?
I would not recommend that. Get a climbing rope.
It would never be my first choice. You could use it in conjunction with a climbing rope as a pull line, thus giving you the full 60/70 meters of climbing rope. However, for canyoneering and/or caving, you'd want a static (as long as you never plan on climbing on it) rope with a water treatment, 9-11 mm. If you're only using it for canyoneering/caving - a climbing rope is overkill as you don't need the elastic properties.
Blue water and Sterling make several canyoneering ropes sold here. My favorite is the Sterling C-IV. Very lightweight and Technora sheath. Has some buoyancy so it helps your rope bag float better. You can also buy an Imlay Canyon Rope but with C-IV available for same price here when on sale the C-IV is a better choice