In the absence of your harmonic injection, the current flowing into mains transformer for the main DC will be a series of pulses, whose width will be dependent on the angle of conduction of the DC rectifiers "pumping" up the reservoir capacitor. Which in turn is dependent on its value and the DC load. The area under the pulse is dependent on the DC current. So one way of reducing the harmonic content would be to load the input circuit when the real DC power pulse is absent, so the input current is a sine wave. This would need a great deal of power to be dissipated. putting in some figures for a 12V 30A PSU, with no reservoir cap, In ~ 12/240 X 30 = 1.5A. With a cap and a conduction angle of 20 degrees, this current would be 1.5 X 180/20 ~ 13.5 A for a square pulse, say 20A for the semi triangular actual pulse. So your kit must "fill in" the missing part of the sine wave. Two triangular pulses per mains half cycle, each 80 degrees long and peak amplitude 15A or so, or a mean current of 80/90 X 15X .5 = 6 A !! So to keep the input current a sine wave, your kit must dissipate 6 A on average with the current being diverted to provide 1.5A real power for the DC output.
It would seem that the best approach is to get the diode on time to be as long as possible as this will result in your kit dissipating less power. The other way would be to reduce the reservoir capacitor but then the output ripple will increase.
Frank