I don't think there is any sci-fi, no matter how hard, that can completely avoid handwaving anything. Take the book/TV show The Expanse. That work is mostly really great about getting the physics of space travel right. But even they have a magic engine that gets ludicrous delta-V, and they don't even try to explain how it works. When you get right down to it, we handwave things in real life even. Nobody knows why bicycles are so stable, but that doesn't stop us from using them.
I think things which are both vital to the plot, but get no explanation are handwavium. How people walk around inside ships in Star Wars is handwaved. Why there is sound in space in Star Wars is not. For something to be handwavium, to me, it needs to be both important AND unexplained, not just unexplained.