For the two categories mentioned above that are plainly illegal, I think, ironically, you can probably just keep AI classifiers running and wait for calls from NCMEC or DMCA emails, exploiting safe harbor clauses and/or by pointing at reasonable measures you've taken. If you have a large post volumes and short public log retention(private logs will probably have to be kept), that would help keeping the pressure low for those.
If 4chan didnt have spam filters, the users would be begging for it almost immediately because the website would become unusable.
What I was trying to point out is that there are a lot of users of these 'edgy' sites that would like to post content that the site owners don't want them posting. So the site owner censors and removes CSAM and piracy so that the site isn't immediately in a legal black hole. The site owners remove content that at least some of the users would want be a use they find it morally or legally objectionable. Spam is kinda orthogonal to that point, since neither the site owner or the users want to see it.
There are forms of moderation that we often dont even think of as moderation or 'censorship' because its removing content that "nobody" wants to see. One may argue that the spammers are users of the site who are being censored.
Also, this is a side note, but I'd assume that the overwhelming majority of even 4chan users are in favour of CSAM being banned from the site, even if there are some users who want to see it (i could be wrong about that, it's not like i even have anecdotes to back that up)