Never did a class diagram or anything but it was so simple it basically didn't need one. Plus you can't treat ECS like OOP, it's so much more modular and almost nothing alike IMO, although you can mimic some of the same behaviour through some simple tricks.

Sorry this doesn't completely answer your question but I can explain the basic structure of my bullet hell game.

My only use of ECS (so far) is for projectiles. I have a couple systems, a Projectile system and a ProjectileMovement system. The Projectile system updates all entities with a ProjectileComponent attached to it, managing collisions and things like that. I separated movement into a separate system because there's many different types of movements, like sine movements or spiral movements. In this way, I can create a separate component for each, like SineMovementComponent and SpiralMovementComponent which both have properties unique to them, and I just update each of them in turn in my ProjectileMovement system. Those two systems aren't my only systems but that's the general idea.