So my question would be is there a way of having a more robust drive system then attaching the wheels directly to the gear motors?
You could drill through the shaft where the wheel overlaps and put a bolt and nylon nut on it. I ended up having to do this with another robot at EXR Manchester to keep their wheels on but it works.

This worked well but as you can see in the video, wasn't all that strong compared to the other robots there. Would it be better to have a chain driven drive for example? Also I have been looking at the P60 gearboxes for more strength also, any thoughts? Andy mentioned Johnson 600s too?
For general power, what sort of voltage are you running this at? P60 boxes are generally quite strong (all metal affairs as I recall) but if you were after a simple solution as the box would need to be assembled, Gimson sell a neat geared motor for about £50 which I've used before in The Honey Badgerquite happily (https://gimsonrobotics.co.uk/categor...tary-gearmotor). That comes with a 775 type motor that has plenty of oomph for the FW class.