Drywall – Best and thinnest way to cover masonry brick wall with drywall

I am remodeling my 1950s home and I have exterior walls made out of masonry bricks (cinder bricks). The interior of the bricks has been painted many times over. I've been given 2 basic recommendations from contractor friends and I thought I'd ask the group.

First off, bricks are porous. I can tell the previous homeowner had issues with this. I'm planning to roll 2 coats of Redgard on the inside of the walls and the outside of the walls will be vapor barriered and stuccoed. Then, my choices are to either place greenboard directly on the Redgard-coated walls or to hang furring strips and then place the greenboard.

I have a couple things to spell out. 2 of the rooms in play are already pretty small. The thought of losing 4 inches of width in these rooms isn't a great proposition (2x 1.5" 2x4s + 2x 1/2" drywall). So, how thin of a furring strip could I get away with?

I live in Southern California, so insulation isn't hugely important for outside cold weather, but should I use furring strips so I can insulate to keep cool air in? Furring strips are also good for running wire, but I don't have a need there.

Any thoughts on my plan? Any thoughts on greenboard on sealed bricks vs. greenboard on furring strips?

Thanks!