Concerning the IMU bias convergence flag, it is only activated once all biases have stabilized. For this, we need to excite the IMU under RTK-fixed GNSS conditions and estimate how the inertial measurements track the movement of the platform. If this is not working as expected, the exerted forces were likely insufficient to fully excite the IMU in all axes.
I suggest making an internal "Maximal" recording on the Vision-RTK2 to fully reproduce what the sensor experienced and diagnose what is delaying the IMU bias convergence. For more information, please refer to Section 5.9 of the Integration Manual.
Another possible cause is electromagnetic interference at the GNSS antennas caused by a nearby USB3 device or unshielded cables. Additionally, if the quality of your correction data is low, the sensor cannot estimate whether the movement corresponds to the platform or uncertain GNSS position estimates. Please ensure you are closer than 15km to the basestation and not in an environment with many reflective surfaces.
Regarding the wheel odometry, even though highly recommended to increase the positioning performance of the Vision-RTK2, it does not affect the IMU bias convergence.