How much Acid are you putting in the feeder? I'd guess a lot, here's why:
The aeration from the water features and the SWCG raises the pH.
The acid feeder responds adding acid to compensate for the increase in pH. The acid also reduces the TA.
You measure reduced TA and respond by adding sodium bicarbonate. The increased TA causes the aeration to raise the pH faster.
And we go around again.
To reduce the acid and baking soda usage, I suggest reducing the TA target to 70 and raising the pH target to 7.6-7.8. Can you adjust the acid feeder?
I'm not so sure that any of this has any effect on the salt level.