It is practically impossible to track when a hero interacts with a garrison.

Hmm, I think that in the case of a garrison, you just need to track the hero's coordinates, because interaction with a friendly garrison occurs when the hero stands inside it. And interaction with an enemy garrison occurs at a different point, yes, but you cannot exchange troops with an enemy garrison. However, Dyrman was working on something similar in relation to a city, but it didn't lead to anything useful because the interactive point jumps relative to the point at which the city's coordinates are defined, and this is due to the rotation. But perhaps everything is fine with the garrison.