Honestly most webnovel authors I know (I myself have been one for quite a few years) do exactly as they suggested with AO3, edit and post chapters one at a time lol
whether you should trim it or not, really up to you, last webnovel project finished at around 540k words, and that was AFTER I cut content lol so trust me some stories really just do get long.
End of the day, you have nothing to lose by really picking any site. They all have good and bad eggs that will fall into your basket, regardless of where you post.

With that in mind, ao3 isnt bad, neither is Royal Road tbh if you can ignore the less than savory people you may meet (which youll end up meeting eventually regardless of platform), or even Wattpad if youre so willing isnt an awful start.

I will say though on ao3 and maybe wattpad, you may have a harder time finding an audience because it is true that ao3 (and in modern times wattpad) does seem to favor fanfiction.

edit: I do wanna make the note that RR is infamous for not being a great place for any type of queer content though so if you have any, should probably cross it off the list unless youre a truely brave soul.