I'd be curious as to what "thin membrane" works out to be R4. It would take about 3/8" of aerogel (not cheap) to achieve R4.
With foil facers you'd still get a bit more heat-rejection against the hot CMU, but yep, a bit of an air gap would improve that considerably. But the benefits taper off rapidly after 3/4-1" of air.
And putting it on the exterior of the CMU would indeed be more effective overall, since the thermal mass of the wall would be on the interior of the insulation.
BTW: Thermax is polyisocyanurate, not polyurethane. The difference is significant, since un-faced iso is both very vapor permeable, and somewhat hygroscopic- it'll take on water. Polyurethane is hygrophobic, and semi-impermeable to water vapor as well.
I'm dubious that a stucco-on-CMU wall with foil faced iso on the interior without at least some gap & venting/weep-holes will make it more than 15 years before holes corrode through the exterior facer. The moisture drives from the exterior will be intense at times, and without a vented gap to purge at least some of that moisture it'll have periodic condensation on the facer which is bound to destroy it over time. Once it's holey enough, moisture will saturate the iso, and the inner facer will start to go, then the condensation & mold potential on the inside of the wallboard will be pretty high.
The builder would likely have retired, changed his name, and moved to another country by the time the REAL fun begins though, eh?
His stackup would work better with XPS than with iso, but with even a half-inch cavity between the CMU & iso, and modest amounts of cavity-venting foil faced iso might make it until his grandchildren inherit the biz.
Prescriptions for foil-faced iso are somewhat different with wooden studwalls than with block. Masonry can handle quite a bit of moisture cycling without deterioration, whereas wood studs cannot. In FL if you placed a highly vapor-retardent foil facer on the interior of the wooden studs they run at much higher humidity levels than if inside the air-conditioned thermal envelope, with a higher risk of rot & mold. But if the stud bays remained empty with no cavity fill or exterior R it could still work with foil faced iso on the interior- it's just a lot less risky to put all of the wood within the thermal & pressure boundary of the structure where it's temp & humidity stays within a safe range.
R6 is a pretty low R value, even for the modest heating loads of FL (or is that the code-minimum in central FL?) For the AC loads the roofing & glazing are likely to be more important than the clear-wall R, but even there R6 seems a bit thin. Your 2" back in the '70s was probably good to at least R11, aged value, and if you filled the stud bays with batts or blown fiber you'd be in GREAT shape for the climate.