What you’re trying to accomplish will definitely work, people use WPO to collapse particles all the time. I’m not able to reproduce your issue with seeing the particles in the shader complexity view, when I use a collapsing material on a default cascade emitter, nothing gets rendered in the overdraw or shader complexity views, they’re empty, just as they should be.
The particles do show up in wireframe view, I’m not sure why. Perhaps WPO is not applied to particles in wireframe (seems to work on static meshes though). I wouldn’t worry about seeing them in wireframe though.
This is what I tested with, in 4.24.1 using the default cascade emitter.