Leisure Time Reserve is sodium bromide while Lesiure Time Renew is non-chlorine shock (MPS). The Renew is acidic so should not only keep the pH in control when you use it but would tend to drop the TA over time as well.
The pH will tend to rise in spas because of carbon dioxide outgassing because the water is over-carbonated. This occurs more quickly when there is aeration such as from spa jets, but even running the pump increases the outgassing compared to still water. Is your spa not covered? If not, then there will be more outgassing as well.
What you can do is to target a lower TA level (as low as 50 ppm if necessary, but with your Renew usage I don't think you need to go that low) and you can use another pH buffer that won't be a source of rising pH. You can use 50 ppm Borates which you can add from boric acid. For 400 gallons, you'd use 15 ounces (about a pound) of boric acid you can get from
DudaDiesel or
The Chemistry Store. Another pH buffer alternative is a phosphate buffer, but that creates a precipitated mess if you calcium is high.
