I've had this one in a few different mixes, none that I cared for. I also couldn't seem to knock out the mites.
I had 8 leaves (it took 2 years) and lost 6 - down to 2 leaves now.
I repotted in sphag and coarse rok in early may - Thought it would probably die anyways
The mites are gone (knock on wood) the roots are popping out, I hope she will continue to do well.
This phal is a bit more tricky & picky then some others.
Grow it in what you already grow
well in. Don't be shy to try different things. You just may find this is the best media yet
But safe is good too
Good luck, Mary