Detached building is more than 30 feet away. So AC (and any other cable that connects) must have a low impedance (ie less than 10 foot) connection to an earth ground for that detached building. Only that (never any plug-in protector) is the only protection from surges.

Of course, a surge is quite rare. Maybe once in seven years. Many do not see one in twenty years. But a surge in that detached garage can also (otherwise) connect destructively to appliances also inside the main building.

Fiber is an expensive option that does not eliminate the need for properly earthed protection. And only does what a MoCA or ethernet cable already does - for much less money.

What is called a MoCA broadcaster, I assume, is an access point. Another solution is a directional antenna on that access point (if it has a removable antenna) directed at the garage office.