Well couldn't you just label a unit type as a
"Priority 1" or "Heavy Infantry" for each nation ?

So you label Abysian Infantry with a Morningstar Priority 1, and Coral Guard, and Centaur Warrior and that accounts for a certain % of the gold usage.

Or use something like Protection greater than 14 (If it's stat based)?

I don't know exactly, but if the limits of deciding gold usage on troops cannot be defined or labeled, what is it doing right now?