@CrystalMethyl so personally I don't read fics but I can give you general advice if you'd like.
1. Everyone starts their writing somewhere, if you look at my first book here and compare it to my current book, you'll see a very stark difference. So don't beat yourself up over your first work not being "good", writing is a process and then more you hone the ability the better it becomes.
2. Personal preference----
As you may notice, all my stories are no ships. Personally I found when I avoid romance the plot becomes more centered around what I'm writing about and there's no issue with building sub plots. Since my stories are all villain/action based, leaving out the romance makes it like a strong dose of that— if that makes sense yk?
3. Grammar.
Omg if you look at my older works you'll see how many times I get dinged on grammar and typos it's so bad. it can take away from a story if there are too many, interrupts the flow. So always check, have someone else check, come back after a couple minutes and check. — things that help catch grammar issues.
4. Know what your plot is, entirely. Know what you are working towards, and the path the story should be taking in the process. When I wrote my first book I went in blind, I had no idea where I was going with it I just had a general idea. That's why it's not as good as my latest works.
Knowing the road you're going to take to the destination makes a book really good, better than when you get lost on the road and end up making some turns that take you away from your original destination.
Plot twists are good, but they need some level of foreshadowing and planning— otherwise they can feel completely random and out of place, like finding a lemon in your soup.