Ok, so it still sounds like you have backbow in your neck tbh, but maybe we just need to go right back to basics. ‘Action’ is basically a term used to describe how high the strings are off the frets. It is totally unrelated to whether the neck is flat or not. ‘Relief’ is a term used to describe the amount of curve in the neck. A flat neck is fine, as is having some curve (relief), but if the neck is bowed backwards you’ll have a bunch of trouble. I’d start by figuring out if your neck is straight or not, and maybe get it straight before doing anything else. You can do this by eye, or you can use a straight edge. I would recommend using a notched straight edge that sits on the fretboard wood and not the actual frets. You can pick them up pretty cheaply on Amazon and stewmac.