Here is a pretty good developer. You need 0.15 g. of Phenidone, 6 grams of ascorbic acid, and 400 ml of borax solution saturated at 20 C. That would be 188 grams of sodium tetraborate decahydrate if you were sure it is the decahydrate, but if there are any lumps in it, you can't be sure, so whether you go through the purification process or are sure that what you have contains only sodium, borax, hydrogen and water, you still don't know how much of the tetraborate you have. Therein lies the virtue of saturated solutions. A saturated solution of borax decanted at 20 C contains the equivalent of 4.71 grams of the tetraborate per 100 ml at any reasonable higher temperature.
I suggest the usual ploy of making a solution of Phenidone in propylene glycol or glycerol. 10 g/l or 1 g/100ml is convenient. It only takes 15 ml of such a solution to make a liter of this soup.
Mix the ascorbic acid and the 400 ml of borax solution with enough water to make 1 liter. Add the phenidone and you are ready to go. What, no sulfite? Yep. Try it without before you add any.
Developing times will be like those you would use for D-76 full strength. A liter of this solution will do 10 to 12 standard rolls if you reuse it. How? The Phenidone is regenerated by the ascorbate, which is in great excess. Phenidone is noy very much affected by bromide in solution. I read many years ago that the activity of a PQ or PC developer would increase somewhat as bromide content rose. I read this in a translation from a Russian paper which was a translation from somebody else, while I was working for NASA, but I did use a similar developer which was much like Crawley's FX-19, though I didn't know it at the time..