What I'm using is the old-fashioned, traditional Sodium thiosulphate. Whether it's anhydrous or not - you've got me there! It's the only type I've ever come across - transparent crystals, elongated with several sides and up to a quarter inch in length, about an eighth diameter. My guess is that it's not anhydrous.
I agree that the amount of hypo is a key factor - too much gives no true blacks and blown highlights, though rating the film at a higher ASA can give usable though low contrast results. When you say that you're using a fraction of the hypo I am perhaps I'm not explaining myself well. I'm dissolving 20g of hypo in 200cc H2O, then using 10cc of this solution per gramme hypo required. At 4g/litre, for my 35mm tank, which requires 300cc of solution to cover one film, I'm using 12cc or effectively 1.2g hypo. How much do you use?
I'm not sure how much the last rinse inversions are necessary, but I guess I'm just erring on the side of caution.
At a rough count-up, I've been through about ten films to get to where I am. Four were with Rodinal as first developer and were at best inconsistent. Then I latched on to the current process, starting with widely varying exposures (and ending up at box speed, as Ilford suggest), then playing with the first development time and hypo added. At one stage everything went to pot and I seemed to be back to square one but identified the problem as a fault with the shutter speeds on my Pentax K2 which, even in manual mode, were pretty much random! That's now away for repair.
This evening I projected some reversal b/w slides to someone else for the first time and the reaction was one of surprise that they could be as effective and were more artistic than similar shots would be with all that distracting colour!