I just plugged your numbers in, and poolmath wants 15 ounces -- just under 2 cups -- of acid to lower the pH. So that tallies with what you're adding. Which makes me question why you posted, "The 2C acid is about 8 times what Pool Math says." Better check the volume you have entered up above. Something ain't right.
2 cups a day is reasonable, until you work that TA down to 60 or 70, when it may stabilize enough for you to only need acid whenever you top off the water. Note that as TA goes down, that the amount of acid needed to effect a certain pH change also goes down. So you can't make a blanket statement that 2 cups of acid will lower pH .5, because that won't stay true for much longer.
I suggest you speed the process up and shoot for 7.2 pH when you add acid. It'll take a lot, but it'll drive the TA down faster. Overall, the amount of acid you'll need to lower the TA a certain amount will remain the same. It's just over how long a period you want to spread it. Daily additions can get pretty old pretty fast, which is why we even have an article explaining
How to Lower TA
In other observations... you ought to raise that CYA to 50 or 60 because the Vegas sun can be fierce and will destroy all your chlorine in a hurry. Also, if you can slow the flow over the waterfall, or reduce the total hours the pump is running, the pH rise will slow down. I run my filter 2 hours a day, and my spillover spa gets maybe 20% of the return flow. It's enough to keep the algae at bay.