The AC resistance of the 28 AWG wire at 10khz is nil. Litz wire is expensive and would not contrubute to increasing the Q at such low frequencies. The MPP cores from arnolds magnetics are "the most efficient cores on the market."
Okay but those are just quoted things, what argument or data support them?

Looking at the curves, I see higher frequency for the same loss, comparing the Sendust 60u of my plot to MPP 60u, say 140mT vs. 250mT at 100 mW/cm^3 and 10kHz. So MPP should be up there with the #2 core on the plot, but with far higher mu, which is quite nice indeed. But also comparable to what you're seeing -- so we can conclude that is currently the limiting factor.
For sine waves, the formula is:
\[ Q = \frac{ \pi F {B_{pk}}^2 }{ \mu_r \mu_0 p_c } \]
Where p_c is the core loss (remember to use consistent units!).
Most of these materials have a core loss best-fit formula with a small exponent of Bpk (near 1), which implies they're better at high frequencies.
A metal has a bulk time constant: \$\mu_0 / \rho\$. This has units of ohms/m^2, so there's still a geometry factor in there. It involves two lengths: the magnetic loop length, and the length equivalent volume of the wire (cross section divided by wire length, as you would use to calculate conductance). So the full time constant is:
\[ \tau = \frac{ \mu_0 l_e A_\textrm{Cu} }{ \rho l_\textrm{Cu} } \]
where the area is the wire cross-section.
The core further improves this, but too much core (high mu) and you can't store much energy before it saturates, and the core properties themselves probably aren't real great (too low R_core).
What you aren't getting, with more turns or higher inductances, is a longer time constant in the copper. As wire gauge gets finer, cross section goes down while length (number of turns) goes up; resistance goes up as the square. Inductance goes as the square too, so they go at the same rate.
Note this means a [physically] larger inductor has a longer time constant. So you can always get better performance with a bigger and bigger core and winding (until transmission line effects stop you)... but, well, that's the rub, isn't it?
Of course you'll need large inductances for very low impedances or very high voltages, and I don't know which way you're going with this. But it's quite possible you're below the peak Q frequency, down in this kHz range. If you can measure at higher frequencies (this will probably require windings with fewer turns, of larger wire; adjust for proximity effect to get equivalent Q) that should be illuminating.
https://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)
This shows how to estimate proximity effect.
Parasitic capacitance of the wire can be limited by the winding technique and can include progressive winding and sectional winding techiques. Progressive winding that is the same from one inductor to another, can be done using toroid winders. My DIY toroid winder is able to use about 3 inch cores and higher. Secontional winding allows the production of multiple inductors with relativly the same winding technique characteristics. I already own 24, 2inch MPP cores that I bought at a electronics depot for a reduced price.
Nice score!

Hmm, 980 turns of 28 AWG is around a 15% winding factor on a, say, C055716A2 (which would only be 70mH); with sectional dividers, that might look more like, 25, 30% per section? So, around 61mm/turn average, which should be around 12.7 ohms DCR? And that's going to be about, what, ehhh, maybe 8-10 layers deep?
Then, proximity effect. h/δ is around 0.4 here (assuming 10kHz), and the plot gives for m=10, Rac/Rdc around... 1.2ish. So if these assumptions are right, it's just getting started, but it's not significant enough to be a problem.
If you're running at higher frequencies after all, or the layers are much deeper (Rac/Rdc ~ 2 at 20 layers), it will get more significant, but shouldn't be a big deal otherwise.

Interestingly, 70mH and 12.7 * 1.2 ohms at 10kHz is a Q of 289, comparable to what the core Q should be. But both can't be this high; the parallel combination must be -- well, if they're comparable, then about half, or 140.
I wonder if I dropped a constant in my core Q formula? I derived it years ago, and no one's told me, but no one ever reads or checks these kinds of things...
Core resistance can be calculated by "Legg's equation"
R(core) = u(r) x L x F (a x B(max) x f + c x f^2 x e x f)
a, c, and e are listed within the arnold's datasheets and therefore the core resistance and be approximated for testing purposes
Notice as the permablity increases, the core resistance increases..That is the main drawback for using solid iron cores.
I'm not sure what all is going on here (two F's? what is the function u?) but that will be the parallel equivalent resistance, no? So a higher resistance is better?
BTW, do you have much experience with PLA or ABS 3d printed hardware for inductor cores?
Yes; it's small either way at low frequencies, but PLA is preferred. ch_scr's link looks useful.
Tim