Okay, starting with the floor and subfloor.
Obviously you’re going to put this on skids to move/put on trailer.
Then your crossbeams to support the building. You can pack an amazing amount of stuff between the cross beams in SDR pipe.
Your floor should float on the crossbeams, makes it easier to pack stuff from above and less portential damage in moving. Box in the crossbeams and heavily insulate around the storage area.
Have I-beams installed for roof/wall support, think roll cage. Depending on what thickness you use, you can bump out the existing framework to deeper set the safe and murphy bed as well as almost doubling the insulation.
You can use the spray in expanding foam insulation, but you’d better have all the wiring and plumbing and anything in place beforehand.
I’d run a couple extra conduit runs in case later you want to expand.
I’d make a solid ceiling using the joists and I-beams, putting a small storage attic in, storage area. The ceiling (floor) would also be heavily insulated.
The roof, heavily sealed and insulated, metal roofs are miserable in hail storms, rain is bad enough.
Cubbys can be built into the walls as you are getting ready to insulate the walls. This way they are insulated in place, making them more a part of the cabin rather than some added hidey hole.
An extra run of heating duct that doesn’t go anywhere, that extra run of sewer pipe, blocked off of course, there are a number of possibilities, but most you will have to include them in original construction.
Except some of the furniture of course.
Depending on size, that ‘combat’ ottoman, fold up daybed frame and such will allmwork. But they all require one thing, you to know what and where to begin with, for sizes and locations.
Too large, you can’t move them around, they block access to other items/locations. Or you end up doubling up on access, the cubby under the storage shelves also covers the access to the underneath piping.
Again, start by mapping and planning the initial build, so you know how much room and access will have to work with.