I tried different mixes two years first one was a well draining one and the second I blended with my native clay soil for better water retention.
The problem wasn’t so much water retention as even water distribution, if you just water from the top the water just wont make it to the bottom without the top becoming way too saturated. If you water slow and long the plants at the top will transpire it all before it makes it to the bottom on a hot day.
I’ve seen approaches where people put a PVC pipe down the middle with holes drilled every so often, then you just drive water down this drilled pipe to evenly water the entire column.
I don’t have a great picture but here is what my ‘vertical planting tower’ looked like:
