There certainly could be better mixes, but straight pumice remains a popular choice among professional collectors.
Am surprised to hear it’s drying out so much - am curious what part of the country you’re in. One way to to preserve moisture is to use smaller particle sizes. Another is to use a mix with some akadama in it near the surface to maintain moisture there. Uncut white sphagnum moss can also help near the surface.
More important may be overhead watering during the day. It’s hard to keep these trees happy while they’re establishing roots without daytime overhead watering.
Shade cloth and moving the tree to a less sunny location can also help.