Consultancies take staff-aug projects (and also sometimes turn them down). It's part of the dealflow.
The easiest way to signal that you don't want a staff-aug gig is to say "no" when you're offered a staff-aug gig. Advanced: counterpropose with a project proposal.
People overthink this stuff, I think.
The one important thing to know is that it's important to be "available"; even if you're not going to pick up the staff-aug gigs, it's probably best to entertain them (and, even more advanced: refer them out to other people), because you can only say "no" 1-2 times before people will stop asking you for anything.
I don't believe "expert consulting", as apart from "freelancing" or "contracting", is a real thing in our industry.