If I'm shooting my daughter's soccer game I follow the action and constantly 'pump' the half-pressed shutter release button to maintain focus (single-point, servo off).
Would re-assigning the auto-focus to another butten be better than what I'm doing?
I would just leave the focus on the shutter button. The M isn't really an "action camera"...however you could try shooting jpeg only, shutter priority set at 1/500, continuous burst mode, a fast card (UHS-1), continuous focus, single point, servo on, and hope for the best
Ditto this. Also if I'm in a similar situation I set fix my aperture as well knowing what kind of DOF it will give me at a certain focal length. Basically I set a mental "focus zone" and just start firing when my subject is in that zone.
It takes a little bit of testing and a little bit of setup, but once you do this, you know that when your daughter moves to a certain part of the field, you're preset to just start firing and you know you'll get good shots. A soccer game is long enough that you should be able to do this a few times from a few different locations. Mid-field, with a goal in the background, and if they let you you can shoot from behind and to the side of one of the goals too. This will give you a good variety of shots.
Again the M is not the best sports camera, but technique can help mitigate its weaknesses.