Well, I guess I'm not sure about mold, but once you get the humidity in the tank regulated a bit more permanently you shouldn't need the humid hide save for maybe when he's going into a shed. If you can get the tank to maintain at the proper 50ish percent the humid hide won't be necessary. Otherwise you could get a big thing of Sphagnum and rotate it out each week - put a bunch in there and swap it after a few days, let the other stuff dry out really well. I haven't used it in my tank so I'm not sure, the only experience I have with sphagnum is keeping it wet for my orchids and Venus fly traps - I generally have some algae after a while, but no mold yet. I am considering making a temp humid hide with sphagnum when he goes blue, but I've only had him for about a week and thus far the damp coco husk mixed with the aspen is keeping the tank at a good humidity.
RE the thermostat, you can find a lot of good recommended brands reading the forums. if you check out the tub/tank setup stickies, I am fairly certain there's 3 brands linked there, a high end, midrange, and a cheaper one. I use the cheaper one, a Hydrofarm heat mat thermostat, it was ~30. It works great, some day I'd like to get a nicer one but for now it keeps my mat at the perfect temp.
Your snake is very lucky he ended up with you as an owner![]()