Update

In the end I just decided to stick with the two teeth rather than splitting them up. Welded the bars in place, but despite trying to get them welded on 180° apart, the drum still seemed to be heavy on one side and attempts to balance it out with angle grinding didn't work too well. So tried the swiss-cheese approach and it now seems to run quite smoothly (still a little bit of vibration but not enough to affect the control)
And yes, I know my welding's pish:



Also welded on some pointy bits (6mm steel) to stop the robot from resting on its side or its back if it gets flipped or does a gyro-dance. The welding was a little better here:



Going to move on to testing now, see if that flags up any weaknesses, and then pull everything apart to check every connector, screw, joint etc. It'll then get re-assembled in preparation for the champs and given a coat of paint. After that, it'll be completely done

And if you work in a confined space, don't spend hours angle grinding bits of metal. Took me three hours tonight to remove the layer of metal dust that had attached itself to everything!