Here’s one more option for you along those lines using stock items in GP…
You can route the outputs of a simple 4 channel mixer to be any of the stereo pairs (1-2, 3-4) per input channel and the output selection of a channel can be mapped to a widget, like a button.
So you could wire the output of 1/2 as stereo to the destinaton 2 channel input, and you could wire the 3/4 output as mono to just the channel 1 input of the destination (summed mono) and then map a widget button to the channel 1-2 output selection (1-2 or 3-4) which would toggle between your stereo and mono paths.
channel 1-2 now outputs to 3-4
