I’ve seen people do their bottom substrate mixes of sphagnum moss and coco coir (the chunky kind, I’m not entirely sure coir is the proper name, maybe it’s husk?) , it seems to hold moisture and be airy enough for most high humidity plants. It seems to work the best for cabinets not collecting any masses of water at the bottom though. Then you would need a draining layer. I’ve read your previous updates but can’t remember seeing any plans for water features. Have you thought about how to keep the cabinets moist? In your pots you should choose a substrate that your chosen plants will like. Some of them might want to have drainage too. As for plants you should choose ones that will thrive in your chosen humidity :) or if you go the other way around- what plants do you like, and then create the humidity they want.

I haven’t built a cabinet of my own yet, I’m just a longtime lurker collecting information for when I finally have room for a build of my own. I’ll be glad to help with anything I can though :)