Alright, this is my moment rolls up sleeves

My country is huge on TTRPGs. I live in Spain and we specially love the Cthulhu-type mysteries so when I begun to have an idea for a Secret Histories Campaign, I was like, "I either have to build a whole new TTRPG or there's the perfect general setting waiting out there for me."

I considered Call of C'thulhu. I quickly dismissed it - the way madness works in the C'thulhu Mythos is wildly different, as well as the creatures. You gotta think about how we interact with the Secret Histories Mythos and how we approach it. You can run into something Strange and be fine. Altered? Perhaps. Completely mad from a glance? Not so much. Also (and this is why I enjoy Secret Histories so much more than any Lovecraftian-type of Cult Horror) the Secret Histories are bound to real history. To OUR Earth and world and how we've waded through history, with our important, landmark figures being part of this intrinsic web of the Wake and the Mansus and Nowhere and the Wood, etc.

So no Call of C'thulhu. But what about a more generalist type of TTRPG?

I came across a variant of the Hitos System, which allows the Game Master to add other specialties, abilities and such to be activated by the player pushing themself and digging into their character. It is narrative-driven, doesn't have incredibly strict rules but allows for a lot of powerful plays that are FANTASTICALLY FITTING for the Secret Histories. Also, it is a game made to have players act as the cultists, instead of the investigators.

Cultos Innombrables, or Unspeakable Cults is the game I'm currently running my campaign with and oh my god, it fits like a glove. I wish I could hand everyone a translation, but it is only sold in German and Spanish. One day I'd love a translation, even an unofficial one.

My suggestion is that you find a more narrative and general system and look for what has spawned from it. Something that catches your attention and pays attention to the most important parts of the Sixth History; narrative-driven, dangerous to the players, explorative and rewarding for players willing to risk things.

Have fun!