I see the improvement in quality which occurs with all the infantry units as the game goes on. But I’m not sure if the Infantry Research option reflects the major reorganization of the German Army which resulted in the formation of additional divisions beginning as early as March or April 1915 (reducing each division from 4 to 3 regiments and adding a machinegun company to each regiment and consolidating the striped off regiments to form new divisions as artillery and equipment became available). I’m not aware of additional divisions becoming available during the game or, alternatively, of additional replacements which might have resulted from these reorganizations.
Since the armies of all the powers reorganized into smaller divisions over time in one way or another I don't see the manpower required to form new divisions reduced as the game progresses. Neither do I see the increasing requirement for artillery/machine guns reflected in the War Supplies needed to form a division as the game goes on. The divisions do get stronger when rested and at full strength as the game progresses. So maybe that's an easy programming solution. But it does not address the missing German divisions.
I've played all the scenarios and the campaign and historical game as the Western Entente, Eastern Entente, and Central Powers several times. I have never seen the internal garrison units of any of the powers become available, other than when attacked of course. Perhaps I have never been in the critical straights the actual belligerent's found themselves in during the war and none of my games have lasted as long as the actual war. So maybe the game allows these units to be released at some late war point? If not this should be allowed, but at some significant cost. These units represent more than garrisons. They were also the training establishment of the various powers and using them at the front would certainly affect national morale as well as the ability to form new units of every type.