Is there an option to change the flight speed of the Mavic 3M in smart oblique? Because when it's quite dark, the automatic speeds are too high

You mean while acquiring multispectral data ?

No, RGB photos

I've been running into the same issue lately with the M3E. The speed in a Smart Oblique mission is locked in auto mode, much like the white balance still is. Which makes it worthless in low light or if you want to just slow down for increased accuracy/quality.