Baking Soda is a bad idea as a pH adjuster, it will cause all sort of issues. Especially since it sounds like you have been using a lot of it chasing your pH. The sodium it contains can cause all sorts of lock out issues, namely Calcium and Magnesium. And without the proper balance of those two pretty much everything else the plant needs is at least inhibited. And the biggest def I see is Cal and Mag which is also leading to P lacking, grey, dying older leaf ends. I would get some proper pH adjusters and flush the res and start over....