I like the system, it is unique and interesting. However, I think it is not optimised just yet. Especially when having levelled your buildings, I often find myself having to click all the tribute carts individually to get them back to my base or some outpost building. Would it be possible to have that Tribute Cart icon near the palantir serve as a global select, instead of selecting each one and cycling through them individually?
Also, I don't think it would hurt to have some kind of hidden timer on the settlements and outposts to see how far off the next spawn is. Would that be possible? If so, I think it would be useful to have this for all buildings that spawn units in intervals, like the Signal Fire, and the Travel Camp, but that is for another topic. It would make things easier to keep track of.
If not, would it be possible, to have some kind of general rally point where all spawned tribute carts will go rally at that certain point on the map? Either individually for each settlement building or some kind of global rally point via the citadel. If you think it would remove the necessary micro, perhaps it could serve as an extra ability for the Witch-King's sanctuary unlock.
Just to make it less arduous to having to micro all those carts in the lategame, that is all.