I agree with what the others have stated.
I have noticed much more aggressive root growth when my Phals are in a fine bark mix in a vented pot. The fine mix stays a little more moist, but not too wet.
It's your call though on the sphag. Maybe you can mix a half and half sphag/fine bark mix? See how that does for you. One thing I can say with my experience is that the sphag breaks down and should be replaced yearly. I do this on all my Pleuros.
I'm actually toying with something different for all of my Pleurothallids.... but I digress....
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