I’d say battening, then insulate between batons, tape up seams and plasterboard. If your not bothered about space you can use bigger stud to fit wider insulation. Up to you.

I would personally Baton inculcate (using 25mm celitex ) tape then board

If you don’t mind losing space build a stud all the way around celotex inbetween the put a 50mm celotex over the lot and foil tape the joints then board. If you want you could put a metal frame ceiling up aswell and put some 100mm loft roll on top and board that aswell that will help the room above a lot

Garage spaces are funny things, they are never intended to be warm spaces so can become complicated to keep warm. You need to think the garage is like a shell and you can't warm the shell but you can build a warm space inside the shell. So what you do is build a inserlation shell inside but it can't touch the outer shell as this will cause the evaporation on the inside so we leave a small cavity between the brick wall and the inserlation so the evaporation ecours on the outside of your inserlation. You will have to connect your timber frame to your outer shell which is fine just make sure to line timber with damp proof wherever it touches the brick wall. After you have made your inserlation shell you can board it all.

NOTE. don't worry about foil tape its not going to be air tight and you won't benefit over the cost you will spend on tape.

Also If your garage has a cavity then you can fit tight against the wall but ONLY if the wall has a cavity space.