commissioning a gamma2 will be 300-350.
Sell the M8 for $600, making your total budget $1100. Get a used Gungnir for $600 (seems to be the going rate) + the USB gen2 install yourself for $100 + Leckerton UHA760 as your new portable.
The gamma2 is great, but not that resolving. I think you'd be happer in the long run going straight to the Gungnir.
Not bad. I like the M8 and want to keep it, I think $500 is closer to the sale price, used gamma2 @ $175 in FS section... I may head that direction, though I'd prefer a gamma2 built by MisterX. Gugnir/mjolnir down the road a bit.