This said, I am making serious progress as one can see.
Sounds good.
I fear will trip me up is balancing.
Balancing can always easily be adjusted in XML at any point. It does not need to perfect from the start.
To my experience there is no need to worry much about its details without having a first demo prototype.
I have no idea how to make the AI actually use the units as intended.
The
"UnitAIs" are available to XML as you most likely know.
(There is XML setup to define the default one, and also for defining UnitAIs that a Unit may chose or not chose.)
Their names are kind of self-explanatory and probably there are already threads with more details around.
If not you could always ask a programmer to post and explain their code once more.
For a programmer it is of course also possible to add further UnitAIs with specific behaviour. It has been done many times.
(e.g. We have added new UnitAIs in our mod several times, when we e.g. added "Wild Animals", "Whaling Ships", "Fleeing Slaves", "Coastal Ships", "Native Traders" ...)
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The more problematic is actualy the
"PlayerAI" logic that teaches the AIs how to use those Units "strategically" (e.g. how often and when to build).
With pure XML settings you usually only get to a certain point, because those mostly only balance the "value" that AI considers the Unit to have.
So if you do
really new game mechanics you normally need a programmer to check the PlayerAI logic and do according adjustments.
(I have no idea how much you have already looked into the PlayerAI logic but some if it is a bit tricky and requires time to understand.)
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I am not sure how many new game mechanics you would like to introduce.
But the more really new game mechanics you want to add, the more programming you will need.
(Both for UnitAI logic and PlayerAI logic.)
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That is pretty much all I could really tell you for now considering AI, mainly because I mod Civ4Col and not Civ4BTS.
Generally however almost everything a modder imagines is technically doable.