Grammar wise I have recently entered the N4 territory. Vocabulary wise I know around 500, but when I was learning Japanese two years ago, I’ve got to around 1300 words, so my guess is I remember 750-1000 words.
Can I do it that I start now but then revise parts of the story to better the grammar to make it sound better?
JLPT isn’t a great measurement for much to be honest. In this case, it would also depend on what kind of story, genre, style, etc. you’re writing in.
However, I’d say you should be good to start output in any form now, including story writing! It may end up being difficult and lead to a lot of mistakes, so make sure to double check your writing afterwards and know what you did well and what can be improved.
Reading lots of native material (especially in the form that you want to write in) will also help in learning how to write it, and what a native would use to sound natural.
Could I create a thread on Bunpro in some category, I don’t know what would fit the most, for the story?
For you to write a story? Or for you to write a story and it sound natural/be easily understandable. If you mean just writing a story go for it now. It’ll be good practice. Realistically it probably won’t be good lol, but go for it. Chatgpt could be a great help, but you should probably only use it for practice. If you’re trying to publish stuff or display your writing ability I feel like AI can be kinda morally murky
There is a recurring theme in several of your posts, indications that you for some reason is apprehensive of simple trial and error, in several aspects of learning Japanese. Making careful considerations for what to do is great, but overthinking it is just spinning your wheels.
The answer to this post’s question is the same as the answer to several of the questions you’ve asked in other posts: Just try it and see for yourself what you are capable of. That’s the only way of knowing. Gaining and building experience, not getting stuck in discussing hypotheticals.
I second @FubuMiOkaKoro and, since you enjoy posting a lot, I would also encourage you to create a single thread to share your thoughts, experiences, learning log, as opposed to a new one for each one of those things.
As for what kind of stories you could write, you could have a look at free short stories here. If you can read them (depending on the level), then you should be able to write with enough practice.
After reading a story there, I think I should not wait with immersion until June 1st, but should start now.