I know nothing in terms of developers. I read that sodium sulfite dissolves grain. Can I modify POTA for more accutance ? I do not care about being able to store the developer fr a long time.
I always thought that POTA gave pretty decent sharpness, since semi-stand is usually given. You could try a tiny addition of Sodium Hydroxide. I've done this with D-23.
Maybe, but adding ascorbate + carbonate will make it into an XTOL-like developer - so no longer the very mild activity of POTA. In fact, if you keep the large amount of phenidone, it'll become VERY active also due to the higher pH of the carbonate. Moreover, because you're removing the sulfite, the shelf life will effectively become zero: the ascorbate will oxidize virtually instantly. This will cause dichroic fog/discoloration on the film.
Why not do the obvious and pick a different developer to begin with? One that just better suits your needs?
Like @koraks said, even a tiny amount of ascorbic acid will vastly transform the nature of the developer because ascorbic acid is superadditive with phenidone.
Moreover, because you're removing the sulfite, the shelf life will effectively become zero: the ascorbate will oxidize virtually instantly. This will cause dichroic fog/discoloration on the film.