Grounding for surge protection
Best option - get an outdoor-rated fiber patchcord, SFPs, and switches with SFP slots for each end. You have the duct in place, this should be simple enough. Prices have come way down. 

If you use the wired method, the wiring /surge device should be bonded to the house and garage grounding systems at the service entrance to each building. Using a shielded cable so you can ground the shield may be beneficial.

Typical cheapest, easiest, good-enough method for most homeowners (since fiber requires replacing your network hardware in most cases) is a point-to-point or mesh 802.11ac wireless link. This may also improve the heck out of your WiFi, depending what you have now. Not as fast as a wire or fiber, but faster than most service to the home.