If determinism is actually how God expresses his sovereignty, then God engineered the plan for history before the beginning of time, wound up the spring at the beginning, and then sat back and let it unwind. To some extent this has to be true, because the effort made to establish energy and mass along with the principles of action that governs their interaction were finished by the end the sixth day of the creation week. Since then, things have generally proceeded on the basis of what was initiated and infused back then. Determinism is accurately descriptive of such governance.
Life, however, throws a wrench into the works, particularly conscious life. Mankind, and at least some of the angels, have the conscious power of choice (will) which, when exerted, affects the details of what happens to stuff in the universe. Life may not be able to change the the spinning of galaxies, but it can affect where the molecules that make up a loaf of bread (or even a mountain) end up today, and whether or not a life continues today. God, of course, can always intervene and interdict such choices, but they are real choices nonetheless, made by sentient beings by allowance under God's sovereignty.
The God who created all things is not under necessity, not determined in his action, but free to do as he pleases, whether this or that. Freewill is not an illusion, then, but the very substance of divine power and attributes which are, in turn, reflected in creation. Freewill exists in nature because God, nature's maker, has freewill and nature reflects his attributes. For freewill to not actually exist in nature would be for nature to fail to reflect a defining attribute of the Sovereign God who created it.