There are two problems I see here that are working against you, one I'm familiar with and one would require the advice of other experts.
First off, why on earth did you borate your pool water so soon after build? By adding 50ppm to your pool water, you have effectively made it A LOT more difficult to adjust pH. If you go to PoolMath and redo your calculations with zero borates, you'll find that it would take only a little over a 1-1/4 cup acid to change the pH by the same amount you are trying to now. I'm not sure if you were advised to add borates so soon after plastering or if you just decided to do it on your own, but that was not something I would have ever advised doing on a new pool.
In conjunction with my first comment, your pool's plaster is only 4 months old. I am not a plaster expert but, if the plaster startup process was not properly performed (and many plaster applicators nowadays do a poor job starting up plaster), then your surface is likely the cause of your pH rise because it is emitting calcium hydroxide (lime) into the pool water. Lime is incredibly caustic and causes large increases in pH. Normally, one would start off plaster using a bicarbonate or traditional startup process where the water is maintained at high alkalinity and pH in order to speed up the formation of calcium carbonate in the plaster matrix. Calcium carbonate acts as a barrier against the emission of plaster components into the water. Usually one spends the first 30 days or so doing the startup process and brushing & vacuuming the plaster dust until it subsides. Then, once the plaster surface is cured and a good carbonate structure is formed on the surface, the plaster is much more inert and has a less dramatic effect on water chemistry. As I said, many plaster applicators do a poor job in this area and they leave it to the pool owner to fight with the pH in the early months of the pool's life. If the plaster was not started correctly, then keeping the TA low is also slowing down the curing of the plaster surface because the carbonate alkalinity in the water is where the plaster gets it's carbonate from in order to cure. As I said, I'm no plaster expert and the startup details matter a lot, but fresh plaster surfaces can take a long time to settle down.
These two conditions (high borates and new plaster) are working against you and you'll be adding a lot of acid for the foreseeable future until the plaster surface cures. If it's not too costly, I would almost advise you to drain the pool and refill it with water that has no borates in it.