Yes, QTR because it’s easier to use for the purpose of maintenance in your case.
In StudioPrint you would have to make a new linearization based solely upon the affected channel. Turn all the others to <none>. Make the affected channel the <Black>. You could edit the linearization table by hand. Drop in 1.3 across all the values. Then make a black image and print it.
However, I do not recommend purging by printing as a cleaning method. It’s not. You have to apply the suction of a POWER CLEAN or INITIAL CHARGE. You have a 7800 and Power Cleans are available. Just use the Power Clean twice to bring PiezoFlush to the print head. Turn it off 24 - 48 hours. Run normal clean cycles until clear. It will waste ink in the other channels. But, it is effective.
Banding however, is usually a dirty damper - especially if the nozzle check is good. Sometimes PiezoFlush will not clear that even after 48 hours. We change dampers on about an annual basis as a preventative maintenance. Hard the first time - and then about an hour or less each time after.