@GillesDB Not sure I fully understand but if you want the X axis to run at a constant speed of 100mm/sec (6,000 mm/min) then you'd need to set the instantaneous speed change to be at least 100mm/sec (6,000 mm/sec). Then there won't be any acceleration or deceleration phases. But because extruders are treated like axes in G1 moves, and all axes have to start and end their move at the same time, then the extruder must have equally high or higher instantaneous speed change. You can set it as high as you like but not lower, otherwise the extruder move timing will dictate the X axis timing resulting in a reduction of X axis speed.
As for changing between extruders, AFAIK G1 moves are carried out sequentially. The first must end before the second can start. That being the case then the only way you can simulate a progressive change in mixing ratio, is to use small segmented moves. The smoothest would be in 1% steps so you could segment that 150mm move into 1.5mm segments with a change in mixing ratio at each move boundary. i.e M567 P0 E1.00:0.00, G1 X1.5 E1.5 F6000, M567 P0 E0.99:0.01, G1 X1.5 E1.5 (the feed rate will be maintained so you don't have to specify it again, nor do you have to specify a "Y" values if there is no Y movement), M567 P0 E0.98:0.02, G1 X1.5 E1.5, etc.....A good text editor or other script would be the best way to generate that section of gcode.
In theory, that should generate a smooth transition between one extruder and the next over the course of 150mm in X without any change in speed (providing you set the jerk value high enough). As with all other gcode commands, you can have M566 embedded in the gcode at the start and end of that sequence if you need to use lower jerk elsewhere.
