At 400 km high the astronauts need to be going about 7 km/s so they 'fall' to earth (inside the ISS which is also 'falling' to earth) in a circle that never results in orbital decay.
Any slower or any lower and orbital decay will take them (and the ISS) into orbital decay and back to Earth.
A platform to reduce gravity by even 50% would need to be nearly 3000km high. Far above Low Earth Orbit where space stations are found.
The base for such a structure would need to be hundreds or even thousands of kilometres wide and it would risk having satellites crashing into it.