first let me express my amazement and my gratitude that you go through all this effort and document not only the final results but also all these important intermediate steps which got you there. There are two variations to this experiment which would interest me personally and which might be of interest to others:
  • Effect of using Borax instead of Metaborate. The latter one is more difficult to get and with a pH of around 8 Borax should be more than alkaline enough to work. It will take a tad more Borax than Metaborate which means we will get better buffering, which in turn means possibly reduced sharpness. Since we turn a lot of wheels here when changing from Metaborate to Borax, a practical experiment could be most helpful.
  • Effect of PG on dev performance. Quite often I see stock developer recipes (think Pyrocat HD, PC-TEA, ...) but need only one or two rolls of film processed, so I'd like to skip the solvent for the concentrate. This would be poor advice with TEA, of course, but what about PG? PG is neither a silver solvent nor an alkali, so leaving it out should have less effect, but from what I read it does interact with the Metaborate. A quick experiment could shed some light on this.

PS: There are a few tricks which could reduce the cost of dev capacity testing: If you use fully exposed test rolls instead of step wedges, you put a lot more strain on the dev, since more silver halide gets reduced. Second, from what I read, T-grain films have less silver than traditional films, so traditional films might again need more dev than TMY2. For testing dev capacity, I'd go with the cheapest traditional 400 speed film you can find.

PPS: Since you design a dev here for very casual home developers, I'm not sure whether we need to squeeze the last penny out of its raw materials. One might as well err a bit on the high side without much financial effect.