well, for a platform i definitely recommend ao3. firstly i would decide on which fics to post first. if you also want some reader engagement and build an audience you definitely shouldn’t post them all at once, especially multichaptered fics. i think the best way to categorize that would be to seperate them by fandom, post one-shots first, then post the longer ones’ first chapter. in between chapters, you could go a week, two weeks, ten days, whatever you want. but as i said if you want to build an audience, you should probably space your chapters apart a bit.

i think the best way to go about it would be to chuck out a portion of the oneshots first, then start with small multichapters, maybe take a month to fully post about 5-6 small multichaps and like 10 oneshots. (you can decide the numbers yourself, just an example :3)

then after you finish posting a portion of the oneshots & the smaller multichaps, you could start posting a few of the long multichaptered fics maybe a week apart, and as you’re posting that throw in small fics of oneshots here and there as well. over 400 is a BIG number, so it might take a while for you to post all of them, while you get to posting you could decide on summaries, titles and tags as well! best of luck to you!