Build a walk-in closet using IKEA PAX sliding door system?

I would rather place the Pax carcasses in the niche and use the versatile practical complement interior fittings. The plinth construction is omitted and the space between the cabinet and the ceiling can be easily covered. If the whole thing should protrude 1-2 cm from the niche, I wouldn't find that so bad.

There are also 3m wide sliding doors.

Milling the groove in the floor is a good idea. You can then install the rail and the slider upside down (so that the guide points upwards). This way you would have no step.