Edward Feser gives St. Thomas’s answers to the dilemma between free will and simplicity (one can more or less adapt the answers from here). Briefly, God does not have a real (or categorical) relation to creatures (since real relations cannot exist without their correlatives) and there is more than one kind of necessity (the necessity of supposition is not contrary to freedom). One can also respond to the dilemma by showing how absolute simplicity and freedom with respect to creation arise from one and the same source: namely, God being pure act.
Pure act is absolutely simple since composite parts are in potency to the whole, and so what lacks all potency lacks any composition. This conclusion is the easier one to see because it demands only that we understand act so far as it is opposed to potency, and thus in an exterior way. The next conclusion- the one concerning freedom- demands knowing what act is in itself.
The fullest and most complete sense of act is immanence, and thus the fullest and most perfect actuality is the most complete immanent activity of intelligence and will. Pure act is therefore immediately and self-evidently personal, intelligent, and perfect with respect to what it wills. This is why Aristotle doesn’t argue to the divine attributes of blessedness or perfect intelligence: as soon as he establishes that some pure act exists he can simply set these things down as what he already meant. This is also why St. Thomas can be so confident in the first way that the first mover is “what all men call God”, since a living, intelligent being with a perfectly rectified will who is responsible for the motion, activity and life of things in the universe is in fact what “all men call God”. (St. Thomas goes on to prove all these attributes too, mercifully, since he realized that his readers probably had less penetration into the concept of “act” than he and Aristotle had).
Pure act thus has a perfectly rectified will, which is to say it possesses the most perfect object in the most perfect fashion, and in an immutable and necessary way. The will of what is purely actual is thus completely determined and unchangeable, considered simply of itself. But this unchanging will is simultaneously absolutely free with respect to its inferiors. Freedom always involves imperfection, even when said of God, for it concerns an object that is other than the last end of the will as perfectly possessed. It’s important to get this right: while freedom is a perfection of the will, it always involves the imperfection, namely the imperfection of an object other than the last end of the will as perfectly possessed. God’s freedom is always said with respect to creatures- for this is the only way that freedom is a perfection of the will. Freedom is indetermination, and indetermination is only of value to a will with respect to things other than an absolute end in which it rests. If you are possessing your last end, indetermination of the will would be an imperfection and defect- just as it is now with respect to the goods we attain (no one wants to possess a good in a way that can be lost).
Our difficulty in understanding all this arises in no small part because we have divinized freedom and thus divinized something other than the greatest perfection of the will as such. We of course continue to want to hold on to goods we possess in an unchanging way that cannot be lost- which belies our divination of our freedom- but we continue to divinize freedom nonetheless.
Pure act is therefore 1.) necessarily simple as opposed to potency; 2.) determined in its will so far as it attains perfectly its greatest perfection; and 3.) free with respect to inferior beings other than its last will as attained. 2 and 3 in no way introduce composition, first because they are consequences of pure act; and second because they are simply two developments of one reality and actuality. They no more divide the will of God and make it two things than the ability to both illumine and dry makes fire two things.