I was traveling sometime around 10 years ago when I waltzed into a music shop and looked around. I saw some neat instruments, and I remember seeing an upright bass that looked just a bit off.
Turns out it was made out of Aluminum and it was made towards the end of WWII to put on ships as a set of big band instruments to entertain sailors. The wood pattern was pretty slick, though just a bit off from what I’d expect.
Invest in really good ventilation!
A lot of apartment shower vents just kinda vent into the wall, with too-small ducting that is easily blocked. I had a contractor get into the walls and put in new wide ducts, making sure they went all the way to the outside vents and were locked on, put new screens to keep bees out, and installed new vent lights with better motors and set them to High. Now I can get out after a hot shower and see myself in clear mirrors, and mildew is no longer a problem. Another thing about OP's setup, it's baths only, and that means less steam in the first place.
If you’re going to consolidate everything into one, don’t half-ass it. Expand the tub, apply a hydrophobic coating to it, and find a mattress that can fit within its confines. Now you have the opportunity to repurpose the top bunk into a photography studio, smoking room, 2-car garage, whatever.
I'm thinking get rid of the tub
Replace with a giant condom, you get in the condom, condom has an inlet on one end, and drain in the other
Squirm around in it to get clean
Maybe the inlet even mixes soap into the water!
That way there is no exposed water/steam/mold