Hello stazi,
regarding unit dependency, i would certainly make them building dependent only. It's an interesting idea to have unique tech trees for each faction, and so have to build your own infrastructure in every province to make it worthwhile in terms of recruitment to have troops from there.

As you say, i'd avoid teh area of recruitment system. It does not work well in TW because the AI is not aware of it, and that is made only worse by the fact that domination is the usual goal. I tried it for modding the medieval game, and the AI was hopeless with it. That was made worse and worse if you took its logic to the extreme.

So sticking to unique tech trees and building dependencies only is the best imo. I personally would avoid the unique tech trees because that means testing all factions with dufferent tech trees and tweak the dependencies to work - and that would be far far more difficult and time consuming with unique tech trees, even if they are culturally unique (ie you have 3-4). But its your mod and up to you - just wanted to make the point for your benefit. As a player of course, i'd love unique tech trees :)