Start with the threshold
Getting into the room matters before anything inside it. Door width, the direction of the swing, and any step at the threshold determine whether the room can ever be used with a frame or a chair. Changing a door to open outwards is a small job that makes a large difference, and it also allows access if someone falls against it inside.
Level access done properly
A level-access shower needs a formed fall, full tanking, and a drain sized for the flow. On timber floors this usually means recessing the former between joists; on concrete it may mean a shallow raised platform with a gentle ramp. Both are ordinary work when planned, and disruptive when retrofitted in a hurry.
Seating, rails and reach
A fold-down seat, rails fixed into proper noggins rather than plasterboard, a shower riser that adjusts easily from seated height, and a thermostatic valve with a scald limit cover most day-to-day needs. Rail positions should be set for the actual user's height and dominant side, not to a generic drawing.
Lighting, contrast and flooring
Even lighting without glare, and a visual contrast between floor, walls and sanitaryware, helps far more than most people expect where sight is declining. Flooring should be rated for wet slip resistance, and continuous rather than patterned in a way that reads as a step.