Thanks for the feedback.
In next version, you'll be able to renew a mercenary contract at a lower cost and for more turns, to give you an incentive to keep that unit alive (maybe). I may add a diplomacy penalty with the original owner if the unit dies at some point.
You may ask, why would one want to keep good diplomatic relations with the Barbarians, so a small preview of multiple tribes placement on the map, open to small diplomatic interaction, not just "Barbarians"
About city production limitation, if materiel is the main factor for production speed, independently from the production yield, there will always be a 100% demand, unless you lack equipment/personnel, and then the materiel cost will never goes down.
(edit: but, yes, maybe use a cap to usage, based on buildings in city or other factors like size/population... edit2: but the AI...

)
I do plan to add sliders to the resource tab in cities, so you can set the % of a resource that can be used for production/reinforcement/transfer/export, you could then limit the materiel usage to control its price.
Trader and Spies doesn't require materiel, yes, Traders are still kind of a placeholder until I manage to include trade route management into the city/diplomacy framework. Same for spies/diplomacy.
Culture doesn't affect border at this moment, only population migration, that will of course change at some point, maybe with Stability. But I'd like to wait a bit as I may overhaul the migration mechanism for performance issues (kind of waiting to be 100% sure the source code won't be released after April before working on that huge task)
Which means yes, good news, I do plan to continue development of this mod, unless Humankind is (much) more open as it seems to be for modding.
Bad news, is, as I don't think we'll get the source, as we can't change diplomacy/AI as much as I'd like, and as even with the source that won't be easy, I'll have to neutralize completely the base game diplomacy framework and start coding my own from scratch, including the AI.
In short, no ETA for next update as it may be long.