For my tank, BBA generally takes off under high light (or light too high for the co2 / ferts dosing) which I believe creates an imbalance. Cutting light back to 6 hrs, getting BBA under control and then working lighting period back up is worth a try.
Except for very high phosphate past test kit limits, the phosphate level has made no difference in BBA growth for me.
Just make a single change, then check so you can see which works.
Instead of adding a buffer which makes a quick change plants have to adjust to, you could try adding crushed coral or a cuttlebone - this will help lift ph in a more gradual fashion.