I use plastic cement three to one with washed sand. If I am doing fireplace boxes or chimneys I use fire clay, it makes a fat mortar. Plastic cement provides air entrainment and contains lime for adhesion. I use a double stucco paper wrap and stucco wire with a full scratch coat. When adding the stone I saturate the scratch coat and the back of the stone, then while wet I parge the back of the stone with the mortar. If the stone is less than 3” wide I add additional mortar with a peak in the middle, wider stones I build up the middle then using the trowel point I make a v-groove in the middle forming a peak along the edges. I give the wall a quick spray and attach the stone.
I also use the 4.5” grinder alot because I don’t have to climb up and down the scaffold so often and the type of stone I am using right now most of the cuts are irregular. I have a small Stanley fan going when cutting.
I think thinset is helping you because of the polymers it contains to hold the moisture. You can get polymer additives also but with good wet mortar and pre-wetting the surfaces I don’t have adhesion issues.
I like that Redgard for showers and bathroom floors, also exterior walls that get a lot of water from splash or blowing rain.
