Easy/Cheap fix to insulating 2x4 exterior wall in Maryland
You can have blown-in insulation added after the fact. They will drill small, maybe 1-1/2" holes in the stucco, one above and one below the fire blocks in each section between the studs and blow in the chopped fiberglass material. then they patch the stucco and if you have some of the original leftover paint, they will repaint it. If not they try to match it, but the success of that varies.

Most HOAs will allow this by the way, so long as the contractor is licensed and in some cases pre-approved by the HOA.

This is what it looks like when they are done, pre-painting.