Suekdccia said:
Consider a halo made up from massive and stable particles like neutrinos* . . . . Those neutrinos in the halo would collapse over time (for example due to the emission of gravitational waves as they orbit the central point of mass) and if the amount of neutrinos is enough this would form a black hole
The assumption in the quote above is not correct.
While it is possible for a halo neutrinos of a given average angular momentum to collapse, there are also ways that the particles in the halo could receive boosts, for example, from the gravity of particles further from the center than they are, and even from self-interactions between particles of the same type. You have to consider all possibilities and there is not a general trend towards collapsing in realistic scenarios.
Also, particles in a halo around a galaxy are also not necessarily in full equilibrium at the outset (which is what the OP analysis assumes). Indeed, in some circumstances, such as a collision of two galaxies within the past billion years or so, they are almost certain to be far out of equilibrium. And the LambdaCDM model of cosmology, which is the baseline assumption despite its known issues, assumes that the main way we get larger galaxies is from the collision and merger of smaller galaxies, in multiple rounds of collisions. Thus, a substantial percentage of the particles of all kinds in the universe, especially closer to the Big Bang, are not in equilibrium at any given time.
Whether a halo of particles around a galaxy is stable is a calculation that can be and is done. If it is stable, one can also reverse engineer how long it takes to become stable and in near equilibrium and how that will affect the angular momentum of the particles in the halo.
But this is a tricky calculation. And, as other responses have noted, it depends upon the distribution of mass-energy and momentum of everything within the galaxy, and not just the halo itself.
As a practical matter, light particles with a low cross-section of interaction, like neutrinos or hypothetical near collisionless dark matter particles, are not very likely to collapse into a black hole at all. There are no known examples of this happening and I'm not even aware of any systems where it is suspected that this has happened (even in cosmology simulations). To do that, you need not just a high enough total mass (the minimum is somewhere between 2 and 3 solar masses), but you need to have that mass to be extremely densely clumped in one place (a sphere of about 12 km radius for the smallest possible neutron star that tips over into becoming a black hole, and larger, as a function of mass, for larger masses).
Getting neutrinos or hypothetical collisionless or near collisionless dark matter particles to clump that densely is much harder than herding cats. And this lack of clumping prevents these kinds of particles from getting enough local density to form black holes in time frames such as the first 14 billion years of the universe.
This is why "hot dark matter" composed of neutrinos or other particles with masses comparable to neutrinos that are almost collisionless (like neutrinos) that have a high mean velocity, were among the first dark matter candidates to be ruled out. Hot dark matter, generically, leads to very little large scale structure in the universe (i.e. to very little matter clumping into galaxies, galaxy clusters, etc.) relative to what is observed.
Neutrinos observed in nature usually have relativistic mean velocities (i.e. mean velocities close to the speed of light). But dark matter candidates should have speeds on the order of a few hundred km/s.
The particles in the halo need to have low mean velocities to be consistent with what is observed, which in the case of low mass dark matter particle candidates (e.g. <<1 MeV) means that "thermal freeze out" scenarios for the appearance of these particles in the universe don't work due to the virial theorem
https://en.wikipedia.org/wiki/Virial_theorem relationship between particle mass and velocity upon "thermal freeze out" of the particles. So, you need to come up with some cosmology that injects these particles into the universe at low energies relative to their masses in dark matter particle proposals involving low mass dark matter particles.
Stacy McGaugh outlines some of these issues in a June 27, 2023 post:
https://tritonstation.com/2023/06/27/checking-in-on-troubles-with-dark-matter/ (under the headings the angular momentum challenge, the pure disk challenge, and the stability challenge).