pic: 4WD Concept

[Chris is me]

First thing I would be tempted to suggest is direct driving the back wheel. You do have to make an asymmetrical gearbox to get everything to package nicely but it saves you a few hex bearings and a shaft in weight. The other benefit of this is that your traction wheel will always be powered even in the event of chain failure, and more of your robot's weight is over the traction wheels.

The solid bearing blocks inside the tube like that really shorten the distance between your two bearings, resulting in a less well supported shaft. I would consider a tensioner free belt drive (no bearing blocks) or some other kind of bearing block like the VersaBlock for adequate shaft support.

If you're willing to step up to 3" tube and shim your bearings out a little you could pop a 24T 15mm wide pulley in the tube to power your omni. This is how we ran a 4WD in 2013. A 27T 9mm wide belt might also work.

Looks good, these are fairly nitpicky suggestions and it largely comes down to preference.

4WDs like this are indeed an option, just know what the trade off is. You're sacrificing a good deal of traction and a centered turning point for better performance against T-bones and a turning point about one end of the robot. T-Bones were a big concern this year with all the defense but they are not always a big problem. It's not inherently harder to drive than a 6WD, just different.