If you want to keep a large garage cool, then your first step is going to be insulation. About ten years ago I did this to my concrete similarly sized garage, and turned it into a 2 roomed workshop.
I insulated with a mix of fibreglass rolls insulation and foam board(celotex etc.). If I were doing this again... I would 100% save up the money and do it entirely in foamboard insulation. It's more expensive per sq metre, but it's so much easier to use and far quicker also, and the degree of insulation can be very good if you use the thicker boards.
If you spend some time researching you can often find reasonable prices on multiples with delivery. Get some proper jointing aluminium tape also.
If you can achieve 100% coverage internally, including the floor, then you will produce a temperature stable interior. Fluctuations outside will affect it inside, but slowly. this means that in the winter, a small heat source such as a greenhouse heater tube will be pretty effective. And during the summer opening any windows during the night and closing during the day will help keep it cooler.
In my workshop, I have a heavily insulated "oven" made from MDF and more foamboard. It has greenhouse heaters in it and an Inkbird style system to maintain a temp. It has no cooling, but during the summer I can maintain about 20c min by opening it up in the night. I also have a large secondhand(cheap) fridge freezer in the room, and if I want to chill or keep low fermentation temps I use that.
Insulation is key......
