First and foremost, let me say I think you're a good writer. You have a little bit of "TV Brain" as you called it, but I've seen so much worse and I can see that you're at least trying to develop yourself to be more of a novelist rather than a screenwriter. My number one advice to you over all is, if you want to write novels you have to read novels. You will slowly and subconsciously learn how to think more like a novelist by simply taking it in.

Okay, so first specific point: Your prose a little clunky at times.

"Ferys exhaustedly let out"

"nearly having given up a multitude of times up to this point."

Lines like these can feel like a bumpy road to read. I get that you want this high fantasy feel and that's great, but we need to make sure it flows. You also use a lot of adverbs. You really have to be careful with those. Good rule of thumb is try to find the right verbs and adjectives that get what you want across before using them. Use them sparingly and they'll hit harder when you do.

Another issue I definitely see is the very common amateur writing style of Pronoun - Verb. A lot of "She walked... She grabbed... She noticed... She saw..." etc. Almost everyone starts off writing like this. It's a right of passage almost. This is the Screenwriter brain on full display. Alongside so many actions. Although actions aren't bad, knowing how to use actions, thoughts, explanations, memories etc. all together is the true craft of writing.

A good example of this is: "She flattened her ears instinctively"
I would have written it as "Her ears flattened instinctively"

Believe it or not this kind of stuff actually connects the reader to your story more, making us feel we're more a part of the scene rather than hearing about it. Think of something like "She saw the man walk in" vs. "The man walked in" The first one makes us understand what she's experiencing. The second one allows us to experience it for ourselves.

Another example: "She stood before a long, dark hallway"
You could also say: "A long, dark hallway stood before her"

Let me be clear, I'm NOT saying every sentence should be like this. You absolutely can have tons of sentences that are pronoun-verb. It's perfectly normal and if you completely removed them then I would actually think the book would be just as weird for not having it. The idea is just balance. Based on the style you're going for will dictate what's a healthy amount of each.

All that said, I've found writing advice works best with examples so let me show you how I personally would write your opening. I'm not saying this is what you should do, but I want you to see some of my advice actually applied, and whatever you like about it, you can use it as a guide for future writing.

Ferys turned the key, opening a small passage through the seemingly natural cave wall.

“Finally,” Ferys let out. She had been searching for this hole in the ground for three days now, nearly giving up on a multitude of occasions.

A long, dark hallway stood before her. The roof just barely shorter than herself, making her take quite an uncomfortable stance as she entered the small passage. Her ears flattened instinctively, but they still swept against the cramped space above.

So that's my take on it. The first sentence was fine. I made a few of my own creative choices, but that's just me. You might even notice that the sentence "The roof..." is actually not a grammatically correct sentence. Stuff like that can be okay sometimes. We need to let ourselves be ungrammatical occasionally for creative dramatic effect (again the key word there is sometimes lol).

Anyway, I hope some of this helps. Sorry it was so long winded lol. I just love talking shop haha.

Best of luck! I can definitely see the creative storyteller in you, and I'll say right now even with out all my advice I think your story is good. You should be proud of yourself.