No. Free will is not an illusion. It is the conscious mind that is the illusion.
What is the opposite of illusion? Did you think that something being real means it can’t be an illusion? Of course not. When you watch TV, do you think what’s being shown on the screen is real? No. It’s an image of something else. But the image itself is real. You’re watching a real TV show on a real TV. The image is real. What it’s depicting isn’t.
Your TV show has characters. It is a real TV show you’re watching, with real characters. But who the characters depict are not real. Harry Potter isn’t real. But he is a real character.
English does not have a word for the true opposite of illusory. I’m not even sure such a word would be even useful. Perhaps it might find use in Buddhist circles.
Free will is the real capability of our conscious minds to choose their own paths and reasons. It’s just that those minds themselves do not reflect anything real. They are images, just like characters and TV screens. The mind depicts situations and scenarios and then moves on. It’s only purpose, like a TV screen, is to provide a substrate for experience.
TVs are amazingly complicated things. They should be, they’ve been the subject of constant industrial innovation since they were invented some 80 years ago. They’re amazingly complicated, yet their only purpose is to create and maintain an illusion.
As is your mind. Free will is just part of the machinery that allows it to do that.