The design would be improved if you provide 2 support point for yaw and roll axis. i.e. additional bearing behind the roll motor and below yaw motor.

Also it looks nice when there is no wires, but much different when tere is. Pay attention to the wiring and maybe reconsider the board location.

Also different camera and lens setups maybe need better adjustability.

Gimbal motors do not usually have solid enough axis to support the weight alone. Also plastic may easily flex. all flex is bad. For that reason it is good to add additional support. On the above design the Pitch axis is supported from both sides. It would be good to have additional support also for the two other axis.

Building from aluminium gives a bit more freedom as it does not flex as much than plastic, but if one side support is used like on my videos, that requires special attention for the motor axis support.

3D printed DIY gimbal could be interesting for many I think, but the above design still needs some evolution. Best way is to start building and then improve.

Hollow shaft is just simply a hole trough the shaft.

I would rather extend the bottom plate behind the roll motor. That would not take any space from the camera.

Also think about the wires and electronics place when designing it, rotate motors so that wire lengths are minimised.

Also consider the needed adjustability for balancing. you could use also counter weight for balancing.

You may want to reconsider also the shape of the parts. the edges could be rounded and instead being solid they could be less weight support structure inside. Think about the bending tongues because of the camera weight on the construction, and make it more solid for that.

Also think about the gimbal moving and fingers. Not creating construction that could potentially hurt the user.