This is the main issue I have with my fic right now.
Enough was left untouched in canon that I have a lot that I want to tackle. It takes me a couple months to really hammer down the chapter and that's even if the original 5k draft isn't scrapped and rewritten.
I have a thorough outline I follow which really helps but as you said sometimes better ideas do come up which throw the original plot in a whole different direction.
Then there's also characterization like I will be obsessing over "don't" or "do not." I'm only majorly dealing with a couple character dynamics but I always sit there thinking "Would they act like this?" or "What if I had them do blank instead?" You have the ability to do anything so it naturally leads to endless possibilities for your brain to think of. What works best for me is writing down specific "lines" characters wouldn't cross. Keep in mind events that shaped them in canon and in your fic. As an example, Canon CRK Gingerbrave may not be in a situation where he has to act a certain way but Fic Gingerbrave may be in a situation shaped by your fic's events where he actively has to. And depending on how connected your fic is to canon, canon events and fic events may hold different weights.
(But also, depending on how long you've known the characters and how much you're fixated on them, characterization comes easily. For example, I don't have to do this as much or at all with Strawberry, but Pure Vanilla may be a bit harder.)
Plot points are a whole separate matter when your fic is lore driven or even not at all. It only takes one oversight to contradict canon or even your last chapter. You have to keep all your plot points in line so you don't forget them or accidentally drop them. For this I personally use a solid outline. Maybe a couple bullet points at the end of every chapter noting what changed between character dynamics or in the plot and where that leads to keep it simple (I think I gave you my full one a while back). I'm going to be honest, unless your plot hole is like HUGE people are probably going to overlook it. Usually you can fix it too (again, unless it's really bad), Gingerbrave was lefthanded in Chapter 2 but righthanded in Chapter 6? Congratulations, he's ambidextrous!
(Plus, CRK specifically has lore that's actively being retconned or added to all the time. You don't know what could change so it's better to just keep trudging forward with the information you have.
My central conflict was solved off screen canonically. But that ends up working in my favor and I don't really have to change my fic as I have free will. I said the hut was most likely not near a beach [really obscure gamble] and that was retconned/added as new lore.)
I don't have the issue where people have already written fics with ideas I wanted to use (not because I'm actively plagiarizing left and right but because the characters I write about aren't popular enough for that to be an issue).
But I know that's a bigger issue with more popular characters. Point blank, two writers are never going to tackle a conflict in the exact same way. Everyone has a different writing style and understanding of a character etc etc. Inspiration is a great thing to have as long as it's not copying.
Do not psych yourself out too, I have regrets even about my most recent works but if anything, it's a learning experience and you can always rewrite.
I'm not an expert fanfic writer by any means nor do I know if this is helpful at all but good luck with your writing twin 🫡
