The sliding door isn't ideal. They do save space, but most people who get them usually find out why most people don't have them! They usually leave gaps at the edge when closed, and you can't creep to the loo in the night silently after dragging the thing along its runners. I do see why it's there, but could you move the doorway further back, away from the bed and have it swing into the room?
I'd try playing games with swapping the new smaller bedroom and bathroom around, see what gives the best all round compromise. If the existing downstairs bathroom is at the back then this may be easier for plumbing. You don't need to be a slave to easy plumbing, but sometimes it can dictate layout, especially waste pipes as gravity only goes downwards. Also check what room measurements you're going to end up with and compare against existing rooms to see how reasonable it feels. It will probably be OK though.
We have a bathroom with the bath under the window. It's not good, you have to stand in the bath to open it. I caught my shorter other half standing on the edge of the bath to reach, had to tell her I'm not pushing her round in a wheelchair after she falls off. Best avoided, but design is always a compromise. If you're going to do it then get a window frame with the opening section and handle at the bottom edge, not near the top as ours is.
Is a loft conversion ever going to be an option? If so then think about it now, as that may involve some changes on this floor to accommodate the stairs.