See the article below to understand how pH and TA interact.With Muriatic acid PH AND TA should go down, right? How long does it take to go down? Several days?
PH TA Relationship - Further Reading

See this article to understand TA and how to lower it. It takes longer to lower to TA than it does pH. So it may take weeks to lower TA from 150ppm. Also, have you tested your fill water? It may be adding TA as the pool water evaporates due to heat.
TA - Further Reading

CH is on the low end of range for plaster pool. Don't let it go lower.What about the CH and CSI?
CSI will change as you lower pH.
With a SWCG you want CSI to be slightly negative