@GillesDB If you set extruder jerk (instantaneous speed change) higher than the speed that the extruder(pump) is running, then there won't be an acceleration or decelarion phase. As the name implies, speed change will be instant (within the mechanical constraints of the system. Certainly, changing the mixing ratio "on the fly" when printing multi coloured objects has never resulted in any unusual print artefacts that I have been able to detect.
One thing of note is that if the M567 commands are embedded in the gcode stream via post processing or custom software, then they will be acted on immediately after the previous move has finished. This might be problematic if you have a long X move and want to change the pumps during that move. You could get around it by segmenting the X move.
Also, if the M567 moves are sent independently of the main gcode stream, then they will be added to the movement queue and only acted on when that queue has been emptied of all previous moves.
Hope that information might be useful in finding a solution that works for you.