What exactly do you mean when you say that the artists who provide it (CP) are not part of the fandom landscape?
BIG CONTENT WARNING: CP, OTHER OBJECTIONABLE PORN GENRES.
First of all, I said that ‘they’ are often not part of the fandom landscape as tumblr knows it. Not always. They can be part of it. The more niche in medium a fanwork is (like, fanfiction has a more narrow appeal than visual artwork), the more likely they are to be within what we typically consider ‘fandom’ in an everyday context. But without going into levels of abstraction about who in the familliar fandom space is socially harmful, you are not going to be able to make a meaningful dent in stopping CP, or graphic porn featuring kids’ shows characters, or transphobic porn, or any number of gross things just by looking slightly to the right in your comfort zone and thinking that is where the problem originates.
The #discourse on tumblr and twitter, outside of like, specifically tumblr or twitter figures, has no bearing on where a lot (but not all! just a lot!) of this kind of art and content actually is spawned. In fact, there’s less of it here than ever before because after the major tumblr restructure many ‘NSFW artists’ jumped ship and no longer use tumblr as a place to host their art. There were whole threads on the gross porn boards on different chanboards mourning the loss, and much of it migrated more completely to boorus like gelbooru or danbooru and other similar sites.
A lot (not all, just a lot!) of these creators are not “part of fandom” as we know it. They have zero contact with your fanfiction and fanart. They are not submitting their work into your spaces, or competing for eyes with your own fanwork, even nsfw or erotic fanwork. They may (not always, just they may) not even have a specific interest in what they are drawing at all, but are merely taking porn commissions. A relatively gentle example: you used to be able to see this more obviously when fandom spent more time on deviantart, where if you searched a character you would see like, fanart... and then you’d see expansion porn where they are inflated like a blueberry or something, and you’d scroll down to the comments and a lot of the time an entirely different set of people would be there, making requests. People who never talk to the rest of fandom, or who do so under different accounts, who never allow the influence of fandom criticism to transfer between their identities.
The people who like, get commissioned to draw bowser destroying the ice climbers, are not going to hear any fandom discourse or in-fandom crusade against CP. They are going to get commissioned to draw more bowser destroying the ice climbers somewhere entirely different on the internet and their artwork is going to be snapped up on a google search or merely reposted to social media in sight of fandom spaces, where rightfully disgusted people will say “ugh! this is happening in our backyard?”
Part of why, when the whole brony porn thing was at its peak (2009-2012?), it was so profoundly fucked up to a lot of typical baseline fandom was that the brony fandom (overwhelmingly born of 4chan culture and part of the ‘manosphere’ at the time) did not obey most of the conventions we expect from typical fandom behavior and so the boundaries between the far sides of the internet were ruptured and people began seeing things they never otherwise would have been exposed to. So if you have ever accidentally seen some fucked up brony art and felt “this doesn’t feel like normal fanart I’d see on tumblr, it feels like... weird porn reposts” that is the reason. The people who made it had a high chance of never caring about, or even hearing about, superwholock, or whatever shipping wars 'fandom as we typically know it’ was stuck on at the time. There was some interest-based intersection... but not enough to share terminology, drama, or a sense of community at all.
In some cases, the artist of this work isn’t even known or may not be someone in the anglosphere because what’s happened is that someone saved a lot of gross pictures from possibly hundreds of remote archives or threads and is just reposting them for blogs/twitter accounts that collect and spread porn, rather than originate porn. Targeting these users does nothing to stop the images’ spread because thousands of people saw the archives and threads the images came from in a completely different part of the internet the discussion has no reach to influence.
Like, obviously, report and destroy truly detestable content when you come upon it. Do not platform or network with a pedophile. Do not reblog from them, or allow your friends to uncritically reblog from them. Do all the good responsibility things to not enable their chain of engagement through you. It is important to make every effort to curate our spaces well and not lose hope or stop caring, or give in to despair like ‘we can’t change anything, why try.’ But that kind of effort exists alongside the truth that what you are seeing is only a feeler of a bigger disgusting critter sent out from a part of the internet you probably don’t go to, and right now have no control over. And pretending otherwise is probably not only ineffective, but spends time destroying our own bathrooms when the real shit is being squeezed out somewhere else.