I like your analysis, but this paragraph is key:
This is the "real world" difference that's not accounted for in your analysis (to give you credit, you admit this). The different movement speeds for shuttling compound the economic impact shown in your data.
Now, previously, some folks said, "Let's just give every soldier the same walking speed. That should solve it!" which I think is not a comprehensive solution, and the "combat balance" of the game, so much as it is, takes into account these walking speeds, so if you did that then the balance would be off again. No, I think the best, comprehensive solution is to have a ShuttleSpeed element for the unit templates. We can set all gatherers to the same ShuttleSpeed, something like 6.5 for the base game. Doing this then removes this problem.
That's because Skiritai are basically Elite Citizen-Soldiers from the get go. They also have a high Walk Speed.
Yeah, we shouldn't want that. So, let's all get on board with ShuttleSpeed!
One of the byproducts of throwing out hard counters is now you have 3 ranged infantry units with no clearly defined roles (their stats are just slightly different and players just figure out which one has the best stats and spams those), so that now you have side issues like this economic question. Let's have ShuttleSpeed so the economic consideration is removed and players can choose units based on their combat prowess and strategy, and not be disincentivized by shuttling speed.