Where should I draw the line between fantasy and realism?

Guys, I'm stuck! I don't know how to balance this issue... there are several things I want for the setting that would never or probably wouldn't work irl and I don't know whether to go for it because it's cool and I am creating a fantasy setting... or if I should stick to things that might actually work. Reason I am asking is because I love things being based in science. Years ago I was completely mesmerized by a mockumentary called Alien Planet where this fantastic alien world was based on what scientists considered possible. So I really want to have things that make sense... but it's also making me spend so much time reading up on tectonic plates, animal adaptations in different biomes, how plants are made up. It's all very interesting and would make the world feel more real, but it's also stopping me from actually making something.

Please, give me some input on the following three things which are currently driving me (the most) nuts 😢

1. I want the sun to be a blueish-white and a contact binary (meaning two stars that are merging). However, blue stars are so hot and technically short-lived, meaning that life on a planet orbiting it doesn't really have the time to evolve. A contact binary star should work but should I change the colour of these stars?

2. The planet, as they know it in the story, is mostly just a vast ocean with three continents on it that takes up around a quarter or a fifth of the planets surface. I was thinking that "the depths" have some hidden secrets and there could be other continents there, but the way I have described it in lore is that no one can pass through them, or at least no one has come back from them. Do I need to have a logical explaination here as to why people cannot pass through them or should I leave it as a mystery for later? Also, another problem is that if the ocean was THAT big then it would create massive storms/waves that would impact the continent which I've made.

3. Floating islands... I love them. But they'd never actually work. But I really want these Avatar-esque floating islands with water falling from them. Can I have those things above AND also crazy things like that or should I dial it back? When does fantasy become TOO MUCH fantasy?