Frank:
5356 a good filler, 4043 reacts with anodizing, and the welds don't look as nice but I use it sometimes with castings as it seems to flow better.
Square wave is generated by some machines, and the purpose is to be able to control the arc profile. I see you have a high frequency output, that is an arc stabilizer. Aluminum tends to buck about 40% of the current in the negative cycle of the ac wave almost like a diode, this means that at a fixed current you are getting 40% less current half of the time, the solution for this is to jack up the current, not good with thin materials. The High freq multiplies the number of cycles to about 600 to close those peaks and give a smoother arc. You hear it as a hiss coming from the machine.
With a square wave output the voltage polarity jumps almost instantaneously from positive to negative without sloping through zero volts as in an AC waveform, the power source has a balance control to increase the time the arc spends in the negative cycle and equalize the arc allowing you to weld very thin materials, and better control at lower arc currents like below 35 or 40 amps.
Its hard to see your machine in the photo, but it would be a control that was labeled "Balance".
Otherwise your welding is "Lookin' Good"
Cheers
Phil