Very little more$ for a hyper heat minisplit, - mine is Mr Cool - - blows away about everything I saw in this thread about heat performance. A 24 K hyper heat minisplit will probably never need any assistance in your garage if you insulate that building like you said. You can always add another 1K unit if you need too. You would probably be shocked at what the 1K would do with your insulation specs in place.
In my 1800 sqft house build in KY I put in a 18K in a large bedroom to make the house livable while I tried to round up a HVAC contractor. It performed so well one day in the teens that I would only need perhaps less that one K to have kept the whole house comfortable.
I installed a 24K in the center of a fairly open floor plan and it does all the house.
We now use the bedroom one for sleeping cold in the summer.
Two minisplits are the cats meow, I am here to tell you. The central HVAC could do nothing to enhance the comfort of my house, wasted all that money on ductwork, wish I had known about the hyper heat minisplit before I did that. And energy use is drastically lower as well. Minisplits don't work in all situations but in a open space like in a well insulated garage