The expanse is scifi, it uses believable extensions of our knowledge, they still have thrust mass, they still have deceleration. Thats probably one of the best examples to use as as scifi actually.
Haven’t consumed the culture.
And if you’ve read Foundations by Asimov and his other books, it’s all explained how the weirdness is an extension of current knowledge (60 years ago). Asimov created the genre, and he’s written nothing that’s fantasy.
None of those use handwaving to explain the “technology”.
Unlike DCC, unlike Star Wars, unlike Star Trek, unlike MCU, etc.
Read Asimovs definitions on what the difference between fantasy and scifi, he’s the one that pioneered the genre and was editor on countless magazines and other written media.