Some observations on brigades
May I suggest combining AA & SP AA brigades. As they are, there is little difference/reason to build AA if you can build SP AA.
In the combined brigade; Model 0 could be AA; and SP AA begin with Model 1 on.
The saved Brigade could be used for Special/exotic weapons add ons.
(Ex. Siege guns, Amph. landing craft, Specific enviroment equipment (winter weather gear (Skis) , Jungle fighting gear, etc); 'Funny" weapons (which would nullify Fortifications); etc.)
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Also regarding Engineers, could you consider including a Logistics Engineers brigade, perhaps tied into the Frontline Supply Tech, which increases max speed of the attached unit. It represents a Transportation Brigade attached to a division to increase its mobility. My uncle served in one such unit it WW2 supplying Patton in Europe in 1944-45. The unit had no weapons (except sidearms & carbines), just 2.5 ton trucks to move supplies speedly.
Stats would look something like this
Logistics Engineer
cost = 0.81
buildtime = 35
manpower = 0.9
maxspeed = +1
supplyconsumption = 0.045
Fuelconsumption = 0.05
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Consider another couple of Police Units. Suggestion:
Model 0: Basic (what we have already)
Model 1: Advanced
Model 2: Special (NKVD, FBI, Gestapo)