Welcome! It would be difficult to be both fantasy and sci-fi as science fiction has a background of science, it will need to follow science based logic. Fantasy by definition cannot be explained by science. Trying to explain fantasy via science is trying to kill the mysticism inherent. You could argue that advanced enough science is basically fantasy, however you will need to do a stringent due process to give rules for your magic like "advanced science" to follow and to keep it in mind for all instances in your book. Well established authors already have trouble following this rule, though they can get away with a few plotholes, they are still plotholes. Easier to just go with science fantasy or fantasy.
However with how history has demonstrated with the likes of Star Wars or Star Trek, the terms sci-fi and fantasy are more based on tropes. If they have bows, elves dragons = fantasy, or starships, lazers = science fiction. So in the end it doesn't really matter, I too have ideas about writing something with both.
