phase knob rather than phase switch

phase swich inverts the polarity of the signal, not really changing the phase. im not sure if reaper calls it phase or polarity. anyway, it would be great if there was a phase knob on every track, especially if it can be in the modulation matrix please.

Constant & arbitrary Phase Shifting for all frequencies introduces latency, no way around the math. The exception is +180 degree shifting, which can be done with polarity inversion as you said, 0 latency. Unless I misunderstood what you meant with "phase knob". There's a JS FX that does arbitrary phase shifting +/- 180.

Open your FX window and type "phase". There are one or two included plugins that will do that. As far as I know, having a phase knob on a mixer isn't particularly common - having a polarity button is.