Potting large Cattleya in low humidity?

[gdupont]

Hi all,

A few weeks back I posted about a Cattleya I was thinking of purchasing, a C. gaskelliana, which I ended up going for! It's awesome, quite large, and it looks very healthy.

The plant came from SBOE, which I assume must have pretty humid greenhouses, as the plant arrived with no media besides a few small chunks of granite from the seedling pot.

I'm wondering how I should pot this up at my home (in New England)? My humidity indoors is only around 40% which is lower than I would like (it will go outside when it gets more consistently warmer), so I think it needs a mix that's a bit more water-retentive, while still being fast-drying. Right now, temporarily, I have it in a slightly larger pot, only half-filled with some new orchid back mix I had available (open to finding a new mix for it online). I don't want to smother the roots, especially if they're used to open air. Am I think about this right?

Right now I'm thinking of getting a bark mix with larger chunks (extra-large?) and maybe throwing in a little bit of sphagnum? Also, probably a smaller pot?

Open to any/all ideas! Images attached.

Thanks!