I would think very carefully before contacting the police. I am not suggesting that you should provide a safe harbor for people sharing CSAM, or obfuscate their crime. You absolutely should take action, but carefully weigh your options before calling the police.

While it may (possibly!) be true in your jurisdiction that platform operators are not liable for user content, police aren’t on “your side”. Even if you assume the highest standards of professionalism from them, they need to represent the interests of the victims (not you) and need to diligently investigate the crime. That means they need to confirm beyond reasonable doubt that you are not involved beyond operating the host.

Just because you self-disclose does not mean that you are innocent. You could’ve been actively participating and when threatened with blackmail you’ve decided to self-disclose to avert guilt.

Another consideration is what else I have on my server. I’m catch and release for pirate movies and TV these days so there’s only 100gb or so. I do have several hundred pirate audiobooks though. Deleting all that before handing my server over will look very suspicious.

With all of this in mind, the only course of action is to talk to a lawyer. A lawyer will know exactly what laws are relevant, and can guide you through the process of self-disclosure while minimising the imposition on you.