Trying to find a way to shut the power off in the detached garage without having to go back to the house to turn off the breakers
This is probably a common issue. The intention is to prevent fire hazards, prevent electricity theft during absence, prevent thieves/burglars using power tools to dismount car parts or invading the main building etc.

An easy, inexpensive and comfortable way can be the installation of a wireless remote (lighting) switch that controls one or more contactors. 

These contactors would feed all the electric circuits in the garage. Normally those sets come with 3 switches which can be mounted anywhere. F.e. 1 secret switch hidden in a trash box or under a table in the garage, 2 in the main building. 

It might be a help to install an outdoor indicator LED lamp ( permanently on) fed by the garage circuit which is clearly visible from the main building in order to see the status at any time.

Even more comfort and safety can be achieved by installing an adjustable  timer (f.e those used for staircases) between the light switch and the contactor(s). No problem, if somebody with memory leakage starts his holiday trip in a hurry, since all electricity in the garage will automatically shut down after 10 min or 2 hours.