Brassia mexicana/signata potting mix?

[tropterrarium]

I have a Brassia mexicana [= signata], that I recently repotted into medium orchid bark plus some perlite. It is kept in Southern California on the window sill, in bright shade but low humidity (it's SoCal).

The older bulbs are shriveling, though new bulbs are being added. I suspect that the potting mix is not retaining sufficient moisture. Googeling for potting mix suggestions, it's all over the map, from bark mounted to hydroculture.

I guess adding peat/sphagnum moss, and adding more perlite, say 30% sphagnum 20% Perlite, 50% medium bark might be a reasonable strategy. Any opinions? Please avoid potting mix abbreviations, as I am not familiar with them. Thanks.