Insulating brick and block walls loosing minimal sq. ft.

R-7.5 and R-5 over the nailers is a far cry from the recommended minimum of R-19 for walls. It's is a homogenous surface and a good thermal break and the Formular won't degrade over time. It would meet your criteria for not taking up much room. The foam sheets will quiet down the house. You'll have to use longer finish nails for the trim work and provide wood nailers around the perimeter of windows and doors, wide enough for your chosen casings.

It can take a bit more care using 3/4" nailers. If the walls are uneven or out of plumb, it's more difficult to shim the nailers while still getting a good attachment to the brick or block. An alternative is to use 2x3 nailers that are more robust, 1 1/2" foam board between the nailers and an additional layer of 1" foam board over everything. You'll lose an extra 1" of interior footage but bring the R-value up to 12.5. This will also give you enough room for outlet and switch boxes. It would be a big improvement over no insulation, more so than just the 1 1/2" sheets on nailers.

The next step is insulating and ventilating the attic where you may be able to get closer to the recommended R-values. Also deal with improving older doors and windows. A small pin hole can let a lot of cold air in, or out when air conditioning. Paying attention to sealing any air leaks is important.

In my own house, I used 1" Formular under the drywall over 6" studs with R-19 fiberglass, also on the ceilings. The house is much more energy efficient than a house with just fiberglass in the stud bays, very quiet. I heat with a wood stove in Maine and have to be careful about getting too hot. I usually only start a fire in the evenings until temps get to 0 or below.  I lose so little heat, I sometimes have to open windows to cool things off, like last night when it was 25 outside.