Given how GR is entrenched now, confirmed so many times, it's pretty clear we're at worst in presence of a Newton-degree of understanding: maybe not all of it,

Given how GR is entrenched now, confirmed so many times, it's pretty clear we're at worst in presence of a Newton-degree of understanding: maybe not all of it, but certainly true within boundaries. Whatever lies 'after' GR can only build upon what GR got right. However in the case of dark matter, let's not forget that a model may be incredibly "right" and yet false, e.g. Bohr's atom; still taught today until you enter quantum stuff, but totally ok (valence etc) for basic chemistry. Newton is also totally ok for small non-relativistic speeds and masses, even though the model is ultimately wrong. Dark matter may also 'work' ok to represent and estimate 'basic galactic motion', but may be totally flawed as a model. We would need to test the stuff itself, not its effects, to validate any kind of theory about it.