-Yeah personally I think I would agree with at least taking the free Stealth 1 Promotion off of National Redoubt, for whatever thats worth.
-In my opinion the Flak Towers and National Redoubts are different concepts. If I'm not mistaken the Flak Towers referenced by the ingame Nat. Wonder are the 8 towers constructed by Nazi Germany in 3 different cities to defend against Allied air raids and actually worked very well, whereas a National Redoubt is a final defensive seat of a besieged government that have sometimes been designated but have mostly not been built or used.
Thus, if I had the goal of reducing the Nat. Wonders available in the Industrial Era I would actually either:
A. Remove the Flak Tower and perhaps roll a small amount of its defensive bonus into the Military Airbase or
B. Nerf the Flak tower down from -100% damage from Air units (thats a lot), and remove its National Wonder classification, its kind of an awkward thing imo to have one city per Civ be basically immune to air damage. Also either give the city its in increased maintenance or just increase the
cost of building them so that they would be built only in the cities that need them rather than all over, this might also be somewhat historically representative, because although 8 were built, many more were planned.
-If one wanted to give National Redoubt a trick besides Stealth 1; I might recommend some kind of a happiness or war weariness buff, since most National Redoubts are never actually used, but our governments and politicians sure do seem happy to have them
.