The "beauty" of a medium mass tonearm is that you can easily add mass, if you decide to go with a very low compliance MC cartridge. Whereas, medium mass tonearms, as is, mate well with most medium and medium high compliance cartridges. On the other hand, you cannot easily reduce tonearm effective mass, if you should want to try a very high compliance MM or MI cartridge from days of yore on a very high mass tonearm. So, I would stick with medium effective mass and think about Triplanar and Reed, primarily, if I were you. I've never heard the Kuzma, so cannot exlude or include that one. fsonicsmith says his Cocobolo-armed Reed is "medium" mass at 18-19g. I'd want to stay in the 11 to 16g range for max flexibility. I use a Reed 2A with a Red Cedar arm wand. Red cedar is a little less dense than cocobolo. But rules like this can be gently broken, because if you look at the equation for resonant frequency, you can see that there is quite a bit of flexibility in values for effective mass and/or cartridge compliance allowed, while still staying in the desired resonant frequency range of 8-12Hx.
As to slate as a plinth material, it's been excellent for me. I own two turntables with solid slate plinths: Lenco L75 and Denon DP80. They're both quite neutral sounding. Slate is in and of itself "constrained layer damped", because that is the nature of slate per se; viewed from the side there are overlapping irregular layers of material, which is what makes it very difficult to cut with a water jet. (I know this first hand.) On my Technics SP10 Mk3, I used a more massive piece of slate coupled to a cherrywood base, mounted to the slate from below. I listened to the Mk3 before vs after I attached the wood base, and I heard maybe a 5% uptick in solidity and neutrality with the wood.
