There is EnTT, an ECS as C++ library:

EnTT is a header-only, tiny and easy to use library for game programming and much more written in modern C++. Among others, it's used in Minecraft by Mojang, ...

https://github.com/skypjack/entt

EnTT is the one I was thinking of -- I'd highly recommend looking into its implementation, as it's both very well-documented and readable, and blazing fast.

The fact that even this caliber of implementation has to handle multi-component cases by doing super careful curation of components is what makes me skeptical that CGS would be able to solve this situation more efficiently/easily. I would be very happy to be proven wrong here, though.

Unity's implementation is well documented and handles this.