Freeze prevention in garage
Your plumber seems to be poorly educated if you are left without the following solutions having been mentioned.

This is  pretty standard problem for folks with a boiler AND a woodstove - the woodstove keeps the house warm and the boiler pipes are subject to freezing off in the corners of the house. Not that this is your situation, but a "standard" solution to "pipes running in a cold space" exists.

The standard solution is a timer that circulates the boiler water every so often (a more sophisticated one could take input from an outside temperature sensor to influence how often) whether or not there's a call for heat. In combination with insulation, that will keep the pipes from freezing.

There are other possible solutions (that don't involve heating the garage if you don't want to heat it), the most obvious of which is to replace the heating system water with non-toxic antifreeze. Heat tape (on the pipe, under the insulation) would be another option, but is frankly fussy and can be unreliable.

A final possible influence - if you use a "setback" thermostat or you manually turn the heat down at night - don't do that in very cold weather. It makes for a long "off" time before the first call for heat after turn-down, and that's when things tend to freeze. When it turned very cold here, I set my thermostat to "hold" and left it there.