Well I guess you are using one of those lnkswitch type things.
The thing is they have an internal fet in the controller, and we have no way of damping the gate voltage turn on…because its all tucked away in the ic.
So we can filter all we like, but we cant make that fet switching any less noisy. You will know that that kind of fet switching creates high dv/dt in the circuit nodes…and yes, this leads to our best loved friend….common mode noise.
And its common mode noise which will force you to re-instate that resistor that you removed from the neutral line….because you need impedance in the loop of current that involves neutral and earth ( I know your circuit doesnt have an earth connection, but every object in the surroundings is connected to earth, so your circuit, like any other circuit, is stray capacitive coupled to earth….at multiple places.
So bring back your neutral resistor so that you have some common mode impedance.
To further reduce common mode noise, you need to make the switching node small, and preferably try and shield it from the surroundings…eg, somehow cover it with “quiet node” copper…eg copper connected to your circuit ground.