Be sure to maintain adequate Total Alkalinity. The maintenance regime you describe (areation and muriatic acid to counteract pH rise) is chemically very close to the method for reducing TA. Pool water with inadequate carbonate alkalinity will etch plaster surfaces even when pH is kept in range. Curing plaster will also cause pH rise but when the cure is complete the pool will need TA addition (baking soda) to offset the effect of the waterfall and negative edge.

Congrats on your pool and welcome to PoolForum!
Sorry for the unsolicited advice, but, it sounds like you're at the end of the plaster cure.