Without the concept of free will in humans, which many now accept as a possibility, what would be the fundamental difference between an entity such as the sun, a plant and a human? None, except the lifespan.
I don't think that was implied, but rather the sun does, what the sun does and we do, what we do. If we do not have free will, but every action, every thought is a determined reaction of the state of things, than we would also be "trapped". But we do not (normally) perceive it as such. We experience our lives and we live it. We act. Even though our actions might come from a deep automatism. For the sun it might be the same, just on a whole different level.
I think the point is a bit more interesting than that. If you instead suppose we don't have free will, and there is no indefinable quality, then suddenly it’s not anymore scary to be a conscious galaxy than it is to be a human.
Right, but you're supposing away the entire essence of the discussion. If you take for granted we have no free will, our indistinguishability from anything else in the universe is an immediately obvious logical consequence.