If you have some way of aerating the pool from the returns that will help you too. At this point, I only have a 250 gallon hot tub on line, but I have chased my tail on getting TA reasonable more than once. There are two ways to do it.. let the pH creep back up over a day or two and hit it again, or take it down to no lower than 6.8 and aerating the water until it's back up to 7.5 or higher and repeat until that takes a long time. It takes hours that way instead of days. Now if that is feasible on a large pool? Well, I'll find out in a couple of weeks. One of the first things I plan on building for the pool is a PVC aerator for a return or three...
My fill water runs 170-180 TA and that is the only way I can get it reasonable for CSI purposes in a reasonable amount of time (and the wild swings don't happen when you are at 80-90 TA either). I was using dry acid and it really didn't work well, even in the 250 gallon tub. Now I literally dose fractions of a teaspoon of 31.25% HCL into a large 64 oz drink glass from a gas station full of RO water and slowly add it into the tub. And the only downside is that the calculators seem to have a fair bit of rounding error (at those small levels), but it seems to be on the "too low" of a dose side, so I have to do this twice each time. I.E. if it shows I need 1.2 teaspoons (I convert the fl oz to teaspoons via Google) I usually need slightly under two...
But acid is acid and you will get it down eventually. You probably have very high fill TA like I do and you need to do this more often or you need to outgas the carbonic acid in the water as CO2. It's a pain to have pH go to low when managing pH, I can attest first hand to that.
By the way when doing this, it is helpful to have an electronic pH meter and use the titration tests only to verify at the beginning and end.