RDoc actually has a good idea... I like it. You could create a passthrough approach with resistors in parallel, each "box" has a different resistor value.
Think extension tubes for cameras. You have a small, medium and large tubes for various macro shots, and you can use any of the 3, or stack 2 of them or stack all 3.
Same would apply with the UMC, the resistor rings, and the modified 14-50 adapter. Without any resistor rings, you'd get 12A (140k ohms), each resistor ring can be color coded to reduce the total resistance when added in parallel to get to the next higher charge rate.
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Or even simpler, create the modified 14-50, create 1 step-ring and a potentiometer, and have labels to the 3 values you need to achieve the 4 cutoffs