My house is constructed of insulated concrete forms. My garage is attached. I have hydronic radiant loops in my garage floor. I have a room under my garage. So...all perimeter edges of my garage slab, and the slab itself are insulated, save the driveway edge.
How do I (or should I?)...
Adding another breaker is easy, but you are correct in that you'll first have to add space. Of course, adding it is only easy if this panel is on an unfinished wall, otherwise you will of course be cutting into sheetrock to run the wires necessary.
Your 60A HVAC almost certainly doesn't pull...