With the earlier adrenaline (most of which ended up on the floor, lolz) I'm still awake. So fiddling with HashSets and trying to put some early optimisations (yeah, yeah, I know) into my game's ECS.
Components point to their Entities and Entities maintain a list of their components (naturally). It means fiddling around with accessors and making things private, adding helper methods, etc. but it's tightening up the code.
Seriously considering implementing IObserver for UI updates as well. Hm.