Add some call mag and I would be on only 1ml grow and 2ml bloom and never had a problem with nitrogen so I doubt it’s that. Magnesium is a mobile element so it would have started from the bottom up. I would add the magnesium at about 1ml per L and see how it goes.
Also just to add I have never used ph down with bio bizz unless ph is over 7.8 and don’t usually water to run off although I will a few time’s over the whole crop cycle.
Lastly using a ec meter with organic feed gives you a rough idea but it will not reflect the true amount of food that the plants could actually eat from the solution once broken down.