If you have radiant floor heat in your garage you are probably using a lot of energy to heat the ground.
I would abandon the radiant floor heat and connect the existing water heater to a hot-water unit heater, or put a through-wall vent or no-vent propane heater in the garage. Then operate the garage at a very low temperature (maybe just above freezing).
An electric micro-boiler supplying the radiant floor heat would probably be the worst possible solution from a cost point of view. If you are going to use electricity for heat, just use an electric unit-heater or leave a lot of lights on.