My purpose in these experiments was to get a practical feel for the results I saw in "Theory of the Photographic Process". Sometime I feel that there might be an advantage in a "thick", less active developer over the thin but very active ones we see a lot of now. My idea was one that would be more or less self replenishing. Phenidone would be a better candidate for that because of it lower sensitivity to bromide build up.

The pH is between 9 and 10 according to my test strips, and it won't change much with dilution because there's a lot of borax in there.

I'm still trying to figure out what you have. There may be as little as 4.5 grams of TEA in a liter that is not neutralized by the ascorbic acid and the sulfuric acid on the Metol. I have a table that tells what pH one might expect.
It says 4.5 grams of TEA/100 ml of solution should give a pH about the same as borax. You have only about 1/10 that amount. It would be a good thing to know if the sulfite was acting as sulfite or just as extra alkali. You could try using 10 grams of borax instead of the sulfite to see what happens.

Cheers,
Pat Gainer