Using lemmy directly will give you more control over the post and lemmy specific features...
Better marketing? Not sure...
And given #xmpp can do things like !movim @lemmy.ml, it's a shame it's not me widely used.
XMPP rooms a generally limited... but you can use the bridging function to access Matrix rooms, which there are more of ATM
https://github.com/matrix-org/matrix-bifrost/wiki/Address-syntax
Misskey, Calckey, Pleroma, Mastodon- All perform the same essential function. Microblogging (with minor differences e.g Pleroma has a 5000 word limit per post) Think Twitter.
Pixelfed is an image sharing platform - Think Instagram.
Friendica will give you a similar experience to Facebook (Macroblogging)
None of those platforms have real messaging (Direct messages, but not private or encrypted)
Almost all sites have an RSS feed...
https://rss.com/blog/ as some advice on finding them...
Even a lot of fediverse platforms e.g Mastodon have RSS 👍
The use of AI (i.e ChatGPT etc) can be a way to help with quickly creating an story outline from your basic idea.
Give it your basic story context, the characters (even if it's just a vague description) and the essential idea of the plot.
The AI will then create a storyboard you can use as prompts for your writing 👍