Try a Ozuku...they can be found cheap...the smaller ones. Excellent finishers. So many options. I use Thuri's , surgical black Arkansas , and jnats. Have fun...good luck
For the money he asks you can buy a labelled escher.. Or a labelled la lune... Or a million different types of jnat... Or a branded norton or pike hard or translucent arkansas... Basically anything with a proven pedigree (provided youbget the jnat from a reputable seller) and proven resale value
Now that's something hard to answer for anyone as now you you have to give your definition of better. Do you just mean finer scratches under a scope, smoother? I mean, a bunch of people would probably rather shave off of things that look worse under the scope than what the 20k gives.
I have used agates.
I got a few different ones and they all give a very shaveable edge but some are better than other.
That small one here is the one that gave me the best result. I picked it up at a flea market in France.