Should I use conduit for a less than 10' run of 6/3 THHN in my garage
If you have evidence of rodent damage to other wires in that location then maybe it's worth doing something more. But otherwise if using conduit is the more difficult method, I don't think I'd bother. If your garage is open framing and you can run from point A to point B in an approved NM fashion, that's the way to go. Nail a plastic 2 gang box to a 2 by and run the #6-3 NM. BTW I think #8-3 NM would've been OK for that circuit.