How I insulated my bare brick walls behind drywall - what actually worked

I did almost the same thing in my 1948 ranch, except my walls are poured concrete instead of brick, so I couldn't leave a gap for drying. I glued 2 inch polyiso right to the block with foam construction adhesive, taped every seam, then floated 1x3 furring strips over it and drywalled. Lost maybe two and a half inches total. That wall used to sweat so bad in January that the baseboards had black spots, and it's been bone dry for three winters now. One thing I'd add is to hunt down the air leaks before you close anything up. I found cold air pouring in around the old outlet boxes on my exterior walls, and a gap where the dryer vent passed through the block. Sealing those with fire-rated caulk where it was near anything hot, low-expansion foam everywhere else, did as much for comfort as the insulation itself. I also skipped the batts in the framed cavity. The foam plus the dead air space behind the studs was plenty for my climate, and I liked keeping the assembly simple. If your winters are harsher than mine you might want more, but for me the difference was night and day with just the foam.

Mine only ate about two inches of depth so I got lucky with the window sills. Yours sounds tighter, so measuring those rough openings first like you said is smart.

Gotta push back on one thing, the 'dead air space behind the studs' part. An open gap between furring strips and foam is not really adding insulation value the way people think. Air in a big open cavity just moves around and transfers heat through convection, which is why real assemblies fill the space or make it airtight. If you're counting on that gap as part of your R-value you're probaly overestimating what you got. Sounds like the foam alone was doing the work on your wall, which is fine, but I wouldn't tell folks the empty space counts for anything.

One thing I ran into after building out my basement walls was that the house got tighter, so the humidity that used to leak out through the block started showing up as condensation on the windows instead. First winter with the new assembly I had water pooling on the sills every cold snap. Running a dehumidifier and being deliberate about venting the bath fans outside fixed it, but I wish I'd thougt about where the moisture woud go before I buttoned everthing up.

Before I covered my block I went over every mortar joint with a screwdriver. Found a couple spots where the mortar was soft and crumbling, raked them out and repointed first. Took a weekend but no way I wanted to chase a loose brick behind new drywall five years from now. I also went with 1x4 furring on the flat with foam between them instead of a full stud wall, kept the whole assembly under an inch and a half deep, which mattered since my room was allready narrow. Saved me from having to shift any door casings.