While I don't have the relevant passage on me, Thomas did say that while God is a cause of every effect, that creatures have their own distinct causality as well. This makes me think that we are looking at something like this: We know from the unmoved mover argument that at every moment in time there is transitive causal power coming from the first cause, to the variety of second causes, and from them to the variety of third causes, and etc. The reason why a first cause is needed is because all of those causes cause in way such that they are only causing because something causes them. This last statement I leave vague because there are two ways people understand this, that the cause is causing them to act just the way that they do, or the cause is causing them to be in such a way that they gain the capacity to do it.

We should view the statement " God is the cause of our choices" as God's causation being a necessary condition for us to make those choices, but not the sufficient thing that makes those choices come about. The causal power such that I can make my choice is due to my deriving that actuality from God acting on me( which happens at every single moment), but the way in which I choose to use that power, or sustain from using that power is of my own volition entirely. God is (a) cause of my choices according to the necessary/sufficient divide, and given that all actuality comes from him transitively, God is (the) cause of my choices since from no where else does the actuality required for it actually come from. Nevertheless I have my own causality and chose freely.