I know it doesn’t sound that exciting, but in between several missions or perhaps in between campaigns your forces should be able to return to a home station and be trained on specifics that you want them trained on. For example, let’s say after the defeat of Britain and before you invade Russia, you return to a home station. What would this look like? There are lots of ways you could do this, but a somewhat simple way might be:
1- It’s a regular map with maybe 8 turns, but there are no enemy units just your friendly core force since this is your base area map that you train on in between your deployments
2- With prestige you purchase buildings, you could pick any number of buildings but for example let’s say a Tank Gunnery Range (Adds some Hard Attack for a Tank) or Bow/Coax Machine Gun Range for Tanks (Increases Soft Attack for Tank), or a reconnaissance course (slightly increases spotting for infantry), a weights center or obstacle course (slightly increases Hitpoints for infantry) etc. etc. you could pick any number of buildings, each for a different branch and relative to a different attribute. Only one unit can be on that range at a time, so I can’t stack all my tanks onto the Gunnery Range all in one turn.
3- You move your units to the facility you want them trained at, then end the turn. Example, I place 1st Panzer Division on the tank gunnery range in turn 1 and then end the turn… at the start of turn 2 it’s Hard attack is 0.25 higher, then I move it over to the coax machine gun range and end the turn, at the end of turn 2 it’s soft attack is 0.25 higher. At the end of the entire home station training, 1st Panzer has +1 Hard Attack, +1 Soft Attack, and +.50 Spotting. You would have to pick numbers that make sense. Once these 6 periods are over, your core force is deployed to the next campaign mission.
The end result is that while a unit’s equipment will be important, a newly recruited infantry unit won’t hold a torch to a highly trained infantry unit that’s been around throughout the war, even if they both have the same equipment sets. I guess overall, I’m just saying their should be a way to increase specific attributes if you want, whether that’s spotting, hit points, movement points, etc. You wouldn’t be able to make drastic advances in one training cycle, but over the course of the war it would pay large dividends for units you have repeatedly deployed and trained.