While dimmensional wood (your planks) are strong, they move or change shape a lot with the seasonal changes in moisture content. Normally, you need at least a 1/2" sheet of plywood to overcome that movement. It must be made with C or better faces (i.e., no D faces), and be assembled with exposure I or exterior rated glue. First, the planks need to be properly anchored to the joists (screws work best), then, when you install the ply, do not install it (screw it) into the joists, but only into the planks. It works best if you install the ply the same way as you'd normally do, across the joists, but place the end of the sheet about 1/4 the way over the joist bay and do not line up the corners of the sheets. no glue under the ply and against the planks, only the screws to hold them together.
By code, a bedroom requires TWO means of egress. Typically, that's the 'normal' doorway and a properly sized window. The window needs to be at least the minimum required by code, and you need to be able to get down to the ground as well.
The floor needs the same minimum ply underneath the tile. Then, assuming you're using cbu, you'd need to bed it in thinset, then tape the seams with the proper mesh tape, then install your tile. The thinset underneath is required to provide 100% support with no voids...voids mean potential movement, and tile hates movement. Often, the floor of an attic is not designed for the load of living space. You should check that first. You may need to add new joists, which will raise it up, and might compromise your overall height. Changing space into living space if it wasn't designed to be, takes a lot of work. Then, in addition to all that, the codes require special wiring accommodations (AFCI), and GFCI (for the bathroom), and smoke detectors that will also trigger the others in the house (and be triggered by them).
Greenboard is generally no longer approved or dictated for damp areas - some codes have not been updated as the localities can choose when or if or which version they approve for local controls. Plumbing will be an issue, as you can't just drain it down the vent from below...you need to connect to the drain down there, and then tie into the vent for the new stuff. Insulation and vapor control is a whole other topic, and will need some review and engineering.