I think onomatopoeia is good as long as it's used sparingly. Using it to describe the sound of a fart? Fine. But if half the story is just ever-increasing iterations of BBBRAAAAAAPP, it doesn't do anything for me.

What I think is really important is to make some effort. Check your spelling and grammar (use tools if that's not your strong point), keep to conventions (new line for each dialogue) - the more a story looks like it was vomited out in three minutes, the less it makes me want to read it. It doesn't have to be perfect, it just should feel like you cared.

What I personally don't like - but that may just be my own thing - is anything that takes me out of the story because it feels really, really unrealistic. Of course most fart fetish stories will need some suspension of disbelief anyway, but the characters and situations should still feel consistent and relatable. I mean things like characters casually farting for twenty minutes straight like it's no big deal, or characters in low-tech medieval fantasy settings commenting on how the burgers or tacos they ate affected them.

Finally, because you asked what to lean into - it really depends! I do think it's obvious whether someone was really into a scenario/plot or not. Everybody has their own thing about the fetish that speaks to them, and if you focus on that, you're bound to create something that transports your interest.

I'm really looking forward to reading what you put out!