The definite positions of your brain states evolution is correlated to the other positions of all the stuff. The other particles do have an effect on your evolution and there is a "you" set of particles one can talk about. Remember the wave function is a function on configuration space so evaluating the guiding effect on the particles is to have to know what point in configuration space it is at; this is actually the troubling bit and leads to the nonlocality concerns, but that problem is common to any quantum theory with definite results happening.

The physics, therefore, is not all in the pilot wave. If you take as the point of a particle theory that there should be particles with positions changing in time, then that is what is being given in Bohmian mechanics.

Also, ask yourself, if the wave function is on configuration space, what constitutes a configuration? In Bohmian mechanics, it is clear, but if the wave function is all there is, then why are we talking about configuration space at all? It is just this abstract vector in Hilbert space evolving and many different representations can happen. Why do we not perceive reality in terms of these other representations?

If it helps, you can think of the wave function a bit like a dynamic law. In [1], the authors suggest thinking of log( psi) analogously to the Hamlitonian H on phase space in classical mechanics. There is no back action on H and most of it is irrelevant to the evolution of a particular particle system in that framework and yet everyone recognizes it as just a convenient way of describing the dynamics.

The difference is that psi evolves but even that may only be true on a subsystem point of view. It is theoretically possible to have a stateless universal wave function which, when particular particle positions of the environment are plugged in, nonetheless gives evolving subsystem wave functions.

Occam's razor is difficult to apply here without a prejudice. If you want to minimize the number of equations, then sure, "the wave function is everything" works, but it comes at the cost of there being what could be considered an infinite number of "you"s and everything else, all slightly different and whole existing other expressions of the universe with no connection to us. If you want collapse somewhere, then you have to posit that mechanism.

On the other hand, by adding in particles and the guiding equation, one gets a singular "you" and everything that we experience is, more or less, definite and singular. So the "existing" stuff is dramatically reduced.

Which one of this is truly simpler is a matter of taste, I would say. I think in terms of communicating with people, the Bohmian version of "there is this universal wave and the positions of stuff are guided by it" is pretty simple. The law itself is so trivially a part of the Schrodinger equation that it could easily be derived before the Schrodinger equation itself. Contrast this with other versions which is "reality collapses to a definite state when we look at it" or "there are infinitely many different universes". None of those seem as simple.

1: https://arxiv.org/abs/1101.4575

Did the Particle Go Through the Two Slits, or Did the Wave Function?