When you are wearing a box on your back, the game tries to compensate the inertia with RCS all the time, rolling out of control.

I would really appreciate a mass multiplier in KIS settings, or something equivalent to have more "gameplay" than "realism".

Or additional RCS thrusters on the box to compesate for the offset mass

The KIS carried parts are equipped in mode "part", which doesn't result in any physics changes.