BUT you cannot say that therefore the less trim lift there is, the more stabilising it is. They are two effects, both caused by the geomtric position of the tail, but NOT linked to each other.
I think you demonstrate the link most eloquently in the previous two lines! I
can say that if I load the aeroplane so as to require a greater (with the correction factors for d_g) lift coefficient from the tail than from the wing, it will be unstable.
Alternatively, if I were to deploy trailing edge wing flaps that (magically) did not move the wing CP, such that a and b did not vary, I would need a very different tail lift to trim the increased CL from the wing, but the stability calculation WOULD NOT CHANGE ONE IOTA.
That case satisfies both your criterion,
and the proportionality criterion, which is that CL*(1-d_g) is greater than CLt. Since CLt is proportional to CL to get the tail lift to the correct trimmed value, if it is stable for the CL where d_g is greatest then it will be stable for other CL values.